It all depends on who you are facing, but I'd say in general range 60cm WB's is the winner of the orbital platforms. I'd always pay for the biggest orbital defense possible though. The platforms can often be avoided or die easily (they can't brace!).
Minefields can't be destroyed and can really scare off your opponent, so they're always a good bet if you have the extra 10 points.
Fire ships are also a bargain - their damage potential is so high they will either attract tonnes of shooting early in the game or they'll be ignored and possibly do lots of damage to your enemy. I've seen a fireship do some serious damage to a battlebarge, as their attack ignores armour and shields.
I'd say anyone who is defending a planet or similar can use defenses, possibly subject to them being appropriate to their race.
Deployment would depend on the scenario and who you are facing. Since they can only reload ordnance, there is no point squadroning them unless they have ordnance.
I don't see why defenses wouldn't benefit from Ld modifiers like everything else. They usually need it with Ld7.