Well i havn't played MW2 or Battlefield very much, but i do play Halo fairly often and funnily enough, if you join an objective based game, mostly everyone actually goes for the objective.
I assume it's because Halo doesn't let you rank up on how many kills you get, it's based on how many games you win so people actually work to do what they're supposed to.
Obviously you'll get the odd person who just doesn't care and just goes for kills, but as long as there are atleast half the team going for the objective, the other people actually end up providing decent support.
So yeah, good work Halo, still going strong and kicking ass.