For realism's sake. Though conversely, this is a game - why not have one?
Why would you need a cover system?
Not exactly like hiding behind a low wall is a brilliant anti-alien tactic...
It was (IMHO) COD4's only weak point.
I disagree, why put something in that has little use? I don't think CoD4 needed one, I think it would have been a poorer game if they implemented a cover system just for the sake of it. They'd have had to have changed the gameplay to take it into account.
These are bitey, chompy, acid-blooded aliens here! they don't fire projectiles. Plus, by the very nature of the creature, it HAS to be a fairly twitchy shooter. Imagine trying to shoot a face-hugger coming at you with a strategic shooter system. I just can't see it
And, if I may, you'd moan like hell if they screwed this game up, as will I, so I reckon Occam's Razor, keep it simple and bare-bones but just concentrate on making what is there as good as it can possibly be. And as true to the films as possible