Apart from compatibility issues, PC users would have an unfair advantage of using a mouse and keyboard in FPS games, although you can get mice and keyboard adapters for the 360 or ps3, it would uneven the playing field for most consol users.
What if you used a xbox 360 controller USB adapter with your PC? That would eliminate the difference in button interface. So I don't see why it's such a big deal to link the two together if it is possible.
there will never be such thing as your saying from this moment, but if there is then the pc version of the game wouldnt be able to play on 360, considering it has old ancient tech. you have to know that pc's are much more powerful then consoles, and simply a pc version of a game wouldn't be able to play on a console platform, because pc games are more demanding on the gpu, cpu, and ram, but maybe in the future there will be