The people that dismiss someone's arguments "because it's coop, not pvp" seem to ignore one crucial thing: there are 4 people in a team, and all 4 need to have fun for the game to have a future. If that doesn't happen, less and less people will play --> the game dies.
Coop or not, there's a meter that tracks contribution. That makes the game competitive. Even though we're not killing each other, most people want to be on top of the list at the end of the round. Or at least not very much behind #1.
Example: high level human vanguard in bronze mopping up most mobs before others get much of a chance to target 'em. Sure, you'll get the same reward at the end, but you're watching someone else go to town for 14mins and that just isn't as much fun as getting some action yourself.
The same can be said for some infiltrators on Silver: while everyone gets the same reward, if you never get a chance to leverage your class' advantages because your targets get sniped all the time, or if you get the feeling you're just an "overload bot" so the sniper can happily headshot whatever is coming in, that too can get repetitive and boring.
Noone likes to be massively one-upped. Not in pvp, but also not in coop.