Well there are probably at least 4 to 6 different forces but i think we should have a choice to play all side for the hell of it!

I don't think that would make too much sense from a story perspective, because the only way to actually win is to stay alive and if there is a battle going on between the S.A.S. and the Spetznas and you could play as both, you technically won both sides of the same battle, which would make no sense at all from a story perspective. The only way that could work out is if the S.A.S. had something like 3 battles between the Spetznas and you got to play 2 as the S.A.S. and 1 as the Spetznas so the S.A.S. could win that battle overall 2-1. But then the story gets too long and most people will be dedicating most of their MW2 time on multiplayer so it would just be a waste of time to do that when they could be polishing multiplayer.