You are caught up in a slippery slope fallacy. http://en.wikipedia.org/wiki/Slippery_slope
'If some chapters flirt with the limitation by taking a few lances, then all chapters will have all lance ships, and will build a fleet that will rival the IN'
They are certainly a long way from that, currently, with only about 12 capital ships in a chapter.
Marines can make escorts, but not capital ships, or at least there is nothing written about it.
First of all, there is no slippery slope fallacy here. There is no middle ground. This is not a reasonable society. If you suggest that the Imperium is incapable of enforcing this limitation now then it stands to reason that they will become less able to enforce it later.
Secondly, even if it were an example of the slippery slope fallacy (which it is not) it's irrelevant. This is because this is how the Imperium thinks. They don't think "Oh hey, what's a few lances? So they could only give us a severe mauling if they rebelled instead of destroying us if they only had a few lances ... Oh, and I'm sure that we won't need to police it, they'll restrict themselves to just a few, it's all good, no need to worry". We're talking about paranoia of clinical levels here. Even if there were some enforcable middle ground then they're still not going to take it, they fear far too much.
If you think that they wouldn't enforce it then you know nothing about psychology. Inefficiency and contradiction has nothing to do with psychology. That's just bureaucracies for you. The people who run those bureaucracies are still human. The fact is, if what you said were true, that the Imperium either can't do anything to stop the SM or couldn't give a fuck then there simply would be no codex restrictions. The SMs would just do whatever the fuck they wanted. Who's going to stop them? They can build warfleets and increase their command to legion status or beyond and carve out their own empire or do whatever. No one can or would stop them according to your logic.
There is rarely a 'line in the sand' in the Imperium, rather a hazy field where, depending on whose in charge of the situation, a decision is made at some point. And a few lances does not an inquisitorial appointment make, read the fluff that has been quoted to you, and it will be clear.
Your current argument is 'Tell me one reason marines would have lances' To which there is a reply, reasons are given, and you reply with 'Tell me one reason why marines would have lances.'
What is the difference between lance escorts and lances on a small percentage of capital ships?
DO YOU REALIZE MARINES ALREADY HAVE AND USE LANCES?
If you keep referring to the psycology of the Imperium, you must know, as I said in earlier posts, that it is incredibly inefficient and contradictory.
Read the reasons given if you feel like it, or give me something besides opinion and conjecture.
The Nova. This is itself a strong argument against SMs getting lances. They already have access to Imperium approved lances in the form of the Nova. Therefore whatever need there might be (and I have seen none) it is already met. As for why the Imperium would allow the Nova, because it's an escort. It's not a threat. Lances on a SM capital ship is a huge threat. On an escort, not a threat. Despite which it is only granted grudgingly and there are factions that don't want the SMs to have even that. What makes you think that it'd be fine on a capital ship when it is so worrying on an escort?
When asked for reasons why SMs should get lances you can't provide any? What, don't want to repeat yourself? You make me repeat myself. So why shouldn't you?
Here, take your very best reasons and list them all in a new reply under "reasons why SM should get lances".
And there are factions that want the space marines to have a lot more (in Armada, even).
Sigoroth, I might point out that your entire argument hinges on your perception of the Imperium. You make two key assumptions: The Imperium is run by ignorant people suffering from paranoia at near delusional levels, and B) They they perceive the Space Marines as the greatest threat to their position.
Let's start with B first:
Which is a greater threat: the admech with their unlimited private fleet, ability to create virus bombs, titan legions, and vast armies of skittari or the relatively self limiting number of Space Marines? Abaddon blew up one star with three Blackstones, however, in 550 M37, a rogue group of tech priests decided to exterminate humanity, and went on a rampage with a similar supernova causing weapon from the Dark Age of Technology for a decade with the near extermination of most of the rimward sectors of Segmentum Obscuris before the Imperium can slip an Emperor Class battleship into the pocket dimension that their leadership is using as a headquarters(WH:40 5th Ed, Pg125)
I would suggest that, by comparison, Space Marines might be somewhat low on the High Lords list of serious threats. Particularly considering that lances or not, with the exception of the Black Templars, a 2000 point fleet, while a fraction of even a sector's IN forces, would be the bulk of a single chapters forces. Not because of the inability to produce ships, but rather the inability to produce more Space Marines. It can, after all, take centuries for a chapter to recover from even moderate losses.
A) The Hight Lords of Terra.
Aside from the Period preceding the Age of Apostasy, known as the Nova Terra Interregnum for the fact that the Imperium was divided in half between the High Lords of Terra and Ur-Council of Nova Terra in Segmentum Pacificus, the High Lords have run the show.
The Lord Commander Militant: Leader of the Imperium's military. This position has, on occasion in fluff, been held by a Space Marine (Guilliman being the first person to hold the office). Paranoid: Maybe. Probably directed outward as much as toward his immediate underlings. Educated: Probably
The Fabricator General: Representative of the Priesthood of Mars. Paranoid: Probably (again, directed at those under him gunning for promotion) Educated: Yes
The Inquisitorial Representative: Name speaks for itself. Paranoid: Yes (and with good reason) Educated: Almost certainly, and in things far beyond most others.
The Master of the Adminstratum: Head Bureaucrat. Paranoid: Definitely of his underlings. Educated: most likely.
I would suggest that while almost certainly paranoid, the High Lords are not uneducated and are, in fact, master politicians, understanding the need for certain institutions to have leeway, even if they're a possible threat to their own power base. For an example of how they have handled things in the past, take the example of Admiral Usurs. Usurs was too powerful to execute without plunging the Imperium into civil war, so he was instead tasked with exploring the intergalactic gulf. After a decade of reports back, he was never heard from again.