Spawning isn't an option. There are only two spawn points at a thime AFAICS, three including airdrops. In Domination and Acquisition I have two options to begin with. These change with the flow of the game, so further on you'll either just be spawning in the middle of your base, or in the attacking helicopter. But you can't spawn to the other side of the map, and would have to run there to join another platoon.
As for their action affecting the play of the level, my point is that could all be simulated. I played a domination match with no regard for what was happening in the other four compass points because there was enough to keep me busy on my end. Yes, I could have wandered over to the other side of the map, but my experience there would be the same, a couple of waves of enemy beat back or not. And if I die over there, I respawn back where I started! The way the maps are designed, there's perhaps at most 32 attackers fighting 32 defenders, or at least a comparable experience. The gameplay experience could be recreated pretty well with a 32 vs. 32 maximum, with Domination happening in 1/4 the size map with 1/4 the people, and everything else that happens in MAG being represented as AI changes to the battlefield based on how well the 32 are doing, or even just a fairly randomised playout.