I like it but only when it makes sense. Clearing an area means you killed all that you found. Surely there are others that can breed or wonder in from off map. Not so many as to be worth grinding but enough to keep you on your toes when you pass through again.

Now the undead (in OS); since you killed the makers of them and all others, there really shouldn't be any regeneration in time. Perhaps there are one or two off map but the guard can take care of that.