Yes. Yes they would. I've seen it firsthand, and not just in Halo. I've seen people go out of their way to do things to get an achievement, even if it comes at the expense of engaging in teamwork. My first MP experience with a 360 game was Gears of War, and it had some ridiculously grind-y achievements of the "Get 100 kills with [insert weapon/attack name here]" variety. There were many occasions where teammates would forgo reviving me even though I was right next to them just so they could get another curb stomp or chainsaw kill or pistol headshot or whatever. When Gears 2 did away with those kind of achievements, the bad behavior that Gears 1's achievements encouraged became uncommon, and people helped out more frequently. Never underestimate the ability of external rewards systems to influence the way people behave.