doors should be able to be damaged when shot with rounds so we could peek through them when they are closed and not breached (destroyed)
(like the doors in left for dead for exemple)

Ins hasnt got a destructible enviornment; this isnt the frostbite engine, it's the unreal engine that sandstorm uses.
I'd be all for a destructible enviornment but it's more work for the devs, and it also ups the requirements for users.

However if you're breaching a room you wouldn't shoot the door and peak through the hole would you cus the defenders would be expecting you and would spray you down instantly, Left 4 dead pulls this mechanic off well because as you guessed it, you're against zombinos who don't have firearms.