Well, i won't go through all the comments, my apologies.
But none, really, there is no point in imposing limits. If that has consequence, though, is an entirely different issue. It does, fps's have changed so much post-Halo that i struggle to find one i actually like. Yet MW2's sales show that from a monetary perspective it works. Anyway, that discussion has been beaten to death already, just a little side note.
The matter of controllers is separate from the matter of platform, all platforms support keyboard and mice and controllers. For some reason, though, most developers don't bother with keyboard and mouse support for their console titles. I'd guess monetary reasons. But it still strikes me as asinine. The only advantage i can fathom is equality in online gaming: everybody has the same controller and the same machine. But even then, there are third party controller developers, even for mice as a replacement to the right analog stick (fragfx, fragenstein, etc).
In my opinion though, keyboard and mouse supplant controllers in almost every genre, except for sports titles (an i mean sports as a generalized term, including motor sports, fighting, etc.). Yes, even 3rd person games, as long as the mouse is used for camera control, and no, controllers are no match for camera control in any way whatsoever, despite what claims can be made contradicting this. But yes, where this is more prevalent is in RTS's, FPS's, and following close behind, RPG's.