This has been something bugging me for a while. I've heard that displacements are cheaper to render than normal brushes, yet they do not seal the map. So what's wrong with simply turning everything that isn't used to seal the map into a displacement? I know there must be something wrong with it, but I'm not sure what it is from a performance perspective. Also, what do you guys consider appropriate times to use displacement brushes? Things like cliffs and terrain are obvious answers, but from what I've seen in some .vmf files, they're also used for tunnels and worn-out roofs. So does this mean displacements should be used wherever there is some geometric imperfection?