Okay, I have worked out a bit from Bullet's post and other posts around here. Like Bullet said, you get XP. But I highly doubt it is for winning or losing only. That is a very unaccurate system.
My Thoughts:
I think it is also based on kills, deaths, betrayals, quitting, completing games, etc. Each positive action would net you XP. Each negative action would lose some. Therefore, you could rank down and such. This is more accurate. So, getting a full lobby to spawn-kill in grifball would serve no purpose. Getting a full team to successfully lock the other team down still would. However, community playlists would probably either net you less, or no XP to prevent boosting. I hope this is true. If so, we can rest assured that the system will be accurate. As long as XP gains and losses for kills and deaths are equal, there is even more hope.
So far, we know each rank you achieve, you earn a set amount of Spartan Points to unlock armor, weapons, armor abilities, packages, etc. Whether deranking to re-earn a rank would net more SP or not is unknown.