Also consider that the original scenarios all have the defenders getting to have orbital defenses while the attacker gets... transports. It seems fairly straightforward that the orbital defenses are intended to do little more than attack transports or perhaps help knock down a shield or two on an exterminator, leaving the fleets that each player brought as the focal point of the game. All exactly as good game designers would imagine.
But of course people wanted more variety than that, as well as a way to represent worlds that can actually afford
real orbital defenses. So we have Space Stations and Blackstones and Ramilies Star Fortresses, but as was pointed out, when those get involved the attackers could need as much as... 875 extra points to even things out? That's LOT of transports.
Have we stopped to think about the fundamentals of what Orbital Defenses should be? Why would you build a defense platform with just one weapon system on it? If you have multiple platforms, but each only has one kind of weapon, then you lose a weapon system entirely if one goes down. However, if you mount all three weapons onto each platform, albeit in lower strengths, you can consistently put a presence against any opponent that comes after you. And in reality, torpedoes and attack craft are the best form of planetary defense. Your opponent must come towards the planet, so the best way to disrupt the enemy on the way in is to fire torpedoes escorted by fighters. This will do one of two things: 1. Completely disrupt the rythm of the attacker as he avoids the torpedo waves and, 2. Likely be taking damage on the way in. After this, I would rank WB's as the next important weapon as most targets would be naturally closing against you at the 60cm range bracket (for those about to say this isn't always true, may I remind you their are ususally turn limits on games that make it more difficult to take the time to line up a platform abeam of you).
This is all fine and good, but there are a few things to consider:
First, as mentioned already, the platforms are not always designed with the best possible budget for the most demanding customers. While various weapon types would present the most versatile platform, they would also present the most expensive and difficult to maintain. Lances and Weapons Batteries are, most of the time, quite different weapons systems, requiring different mounts, different power sources, different ammunition, different crew, etc., not to mention the various types of ordnance. Although on paper it seems as simple as, "Hey, I can get a hit on an escort with Firepower three weapons batteries or a strength one lance, so they're interchangeable," in the BFG 'reality' there are many more constraints to consider.
Perhaps it is not by chance that GW came out with rules which practically highlight the
exact versatility you describe (the Ramilies and Blackstone entries even make a point of describing how they are nigh-invulnerable to attack), and they are universally for large powerful orbital platforms?
Second, you say it is better to have each weapon system represented on each platform rather than having varying focuses. I think this is off from both fluff and gameplay perspectives, and for the same reason each time, surprisingly. As far as gameplay is concerned, most of the time you will know exactly whom you are going up against; their race certainly, if not their exact fleet composition! So why would it behoove you to take something that you know will not be maximally effective against what you expect to face? Have you ever actually played a scenario in a tournament against a random opponent that involved you controlling planetary defenses and defending something? A very unlikely scenario indeed, in my experience. (See what I did there?
)
From a fluff perspective the same thing holds true. A random Imperial agri-world is not going to invest in planetary defenses. Why bother? The chances of the planet being attacked from space by an enemy they can actually hope to drive off with a few small weapons platforms is essentially zero. If a Tyranid Hive fleet shows up unannounced or they pissed off a local craftworld or an Ork space hulk drops out of the warp above their planet they're dead meat anyway... We're not talking Imperial Navy fleet bases or Forgeworlds here, most planets are unimportant and out-of-the-way, not central hubs of starship travel.
Of course, if they've already been attacked, that's another story. Now they know not only that that there is a good chance they
will be attacked (it's hard to forget where you put that nice little mining planet) but they'll probably have a pretty good idea who will be doing the attacking. Those pirates on the fringes of the system aren't just going to give up on the first try, and it's likely more effective to buy some weapon platforms and a monitor or two from the next nearest hiveworld than wait for the Imperial Navy to show up six hundred years later and hope for the best. But those pirates likely only have a motley collection of converted escorts and transports, it's not like the Planetary Governor never knows what heavy warships they'll be throwing his way next. Whether he has Imperial pirates, part of an Ork empire in the next system over, an Eldar webway portal somewhere, or Tau merchants with a really pushy Water Caste, the Planetary governor is rather likely to know what he'll be up against. So again, why buy spend oodles more on a system that may be effective against something that is likely never to appear? Humanity tried that solution and the Imperial Navy was the best they could muster.
Third, I'm a bit skeptical of your assessment of what constitute the ideal planetary defenses. (Although since I've actually used planetary defenses a grand total of, I think, two or three times, I'll certainly cede to your presumed expertise in this area.)
Are torpedoes really that powerful? Sure a S6 salvo from a 30 point platform seems effective, but they
do have to reload. Unlikely as it is that the rest of your fleet will never fail a command check, how many of the platforms will reload reliably each turn? Second, what if you're up against Ordnance heavy Tyranids? Tau? Or Eldar, heaven forbid? Those torpedoes will not be particularly useful unless you wait to fire until the enemy is close enough that they can't dodge, and by then you're basically in range for the other types of platform. Torpedo platforms aren't the only ones that can take advantage of knowing where the enemy is heading, and ask how many Imperial players still launch all of their torpedoes in a big volley from long range. The new FAQ's nerf of torpedoes (what's the deal with that, anyway?) certainly works against them as well, dodging those tiny S3 markers is not difficult and mildly annoying at best.
And... I forgot what my fourth point was. How embarrassing.
All that said, I certainly don't think your idea is without merit. After all, in many situations the type of platform you describe
would be ideal, there are many worlds that are beset by a number of foes, rich enough to actually attempt to defend themselves, and still not so important that one of the many Imperial organizations with enough influence to secure proper protection care enough to actually do so. (For some reason shrine worlds and the Ecclesiarchy keeps coming to mind for this type of scenario, but I'm not sure why.) I could also see this type of platform being valued by worlds that want to support larger defenses without a large number of smaller ones and are looking for something more efficient in terms of coverage. I wouldn't argue that your type of platform should not be an option, just that it shouldn't be and would not make sense to be the
only option.
We allow lock on too 
it seems somewhat silly that they wouldn't try to AIM at the ships moving towards them with ill intent.
I see no reason for anyone not to be able to use the defenses, I'm pretty sure even the nids can spawn something that is able to fire a gun on a captured platform and if that doesn't float your boat then give them spores instead of shields/turrets and make some suitably awesome models to represent them!
Since Lock-On does not represent aiming in the first place, it doesn't really seem silly to me.
Lock-On represents ships diverting the energy from their motive systems to their weapons systems - orbital platforms have only rudimentary motive systems, so what exactly would they be locking on
with? Or are they just letting a bunch of generators sit around idly in case they need to turn it up to 11?
It makes as little sense as letting them use AAF, CTNH, or BR, it's just a lot more obvious with the movement special orders. (As for something like the Ramilies, it
does have a lot of extra power just sitting around, but it's even capable of warp travel with that incomprehensible reactor, so...)
Reload Ordnance makes sense, obviously (how else would they function?) and I could certainly see Brace for Impact making sense. One could argue for the status quo, of course, as these platforms typically don't have experienced captains issuing orders to a drilled crew who can all do things in unison, but it's just as likely they might, and still not much of a reason for them to be without even basic safety and damage control systems.
I think only RO and BFI are really appropriate for simple defenses like the platforms listed in the 'Ships of the Gothic Sector' list, even the space station. I would argue it's even proper for the ships in the defenses section to be unable to use special orders aside from those two as well. Defence Monitors are minimally mobile in the first place, how much spare power can they really have after cramming all that armor, shielding, and weaponry on that tiny hull? System ships are barely capable of combat as it is, they're certainly not zipping around with overcharged engines spitting out rapid-fire salvos. The Ramilies has its own rules, obviously, and I think the Imperial Blackstones should be able to use LO as well considering their background, but those are special cases of unusually and inordinately powerful creations.
As for the Tyranids, I wouldn't consider them using captured platforms, they don't really interact with non-Tyranids, or even other Tyranids, in that way. It could happen if a genestealer cult was involved, but that would be the exception rather than the rule. (See what I did there?
) Although I applaud GW's idea of extra ordnance markers as the most elegantly appropriate solution without creating more rules clutter, specifically spawned planetary defense organisms would certainly work well!
I feel like the fireship option for escort drones would really fit better in this category. Mega-spore mines, purchased in the same way as regular mines, would also be an appropriate choice, although I never understood why they give away VPs for carrying out their exact function. Other things along those lines would be wonderful. I don't know about pyro-acid or bio-plasma 'platforms' so much, but a high-orbit tyranid defense that has launch bays and/or torpedoes would fit quite nicely. Another fluffy/neat solution might be to remove ordnance limits for tyranid planetary defenders, or (perhaps more reasonably) triple or quadruple the launch limits for ships within a certain distance of the planet. Or have them be automatically reloaded each turn regardless of other activities? Bonus spores perhaps?
I like the idea (sort of what them using defenses points for extra ordnance gets at) of the Tyranids themselves being strengthened by controlling a planet rather than using separate special gribblies.
I love a good pun.
