Marines in the game of Halo 2, do not suck. Many people in Halo 1 killed them for ammo or for the heck of it. In Halo 2, Marines are much more lethal, and unless you are playing on easy, have more health than you do.
Here are some good reasons why they are good:
1) The better weapon you give them, (sniper for example) their aim improves drasticlly, and they also become much more 'tactical'.
2) They live by the 'Suck it up marines' and will willingly take hits for you to get in close.
3) They say funny stuff, and a couple things not often heard, (like the famous grunt saying, 'can i have his helmet?')
4) They can drive things for you. On the levels were you kill the prophet, i recomend not letting them drive the Hog' becuase they will most likely drive you off a cliff.
5) I can go on and on and on, but the BEST reason for marines-
THEY WATCH YOUR BACK
In a firefight, they have constantly proven themselves by on heroic, i was pinned, a elite was hammering me, and a marine snuck up,(actully went around a corner but oh well)
and shot the crap out of him with her shotgun.
I think they operate better than most players do, becuase on Co-Op, they just rush most of the time. On multiplayer its a whole new ball game, but the Marines are a Invaluble part of the game.