i dont know the numbers, but i would guess it might be cheaper for MS to toss money at 3rd parties for exclusives than it would be to fund a first party studio. it's certainly safer. timed exclusive or dlc... if the game fails, not a huge loss.
i still say the focus of the 360 is on games. the extra stuff, is just that, extra. also, as the lifecycle of this generation goes on, MS probably thinks there needs to be more reason to attract non-gamers to the system. and if they get non-gamers, they can turn them into gamers. and that's alright with me. the more gamers the better.