I've never heard anyone having it installed permanently (maybe I don't understand what you mean). They are attached by chains and swing a little (as on our previous coach). I sure wouldn't want to remove it and then reinstall on a regular basis.
Both of our RV's had/have the mud-rock guard. On the new coach, the rock guard is installed closer to the back bumper and is more rigid. The car seems to be protected pretty good. We do have a few rock chips on it so I don't know if it happens while driving or towing or both.
MM.