I rarely use the cover feature directly anymore. In my experience, it lowers the firepower of a unit and can compromise their melee abilities as well.

First, units don't seem to fire by ranks. Although this protects them from enemy fire, by not exposing all ranks, it kills their firepower in many ways. The best defense against an infantry charge is a well timed volley from each rank. No-can-do behind walls.

Second, the wall appears to offer no melee advantages at all. If allowed enough room to run, both cavalry and infantry can jump straight over walls with little or no slowdown. Units walking or moving at angles can get stuck or slowed down by walls but straight on charges work pretty well. Especially when the AI does it. Cavalry charges are especially bad with walls, because you can't use a good volley to knock half the unit out.

I sometimes station units behind walls but never use the cover feature.