I use walls and crates as cover ALL the time, what are you referring too? Just make sure the walls and crates are thick enough to stop a Barrett .50 cal sniper rifle as a lot of high level players have them already!
I have not completed the "shoot someone through a wall/cover" challenge yet and I shoot at them all the time (I only have the starting sniper rifles though). They can definitely be used for cover, just choose the right ones.


