Instead of respawning monsters, I'd rather have random encounters a-la Fallout 1&2 where you can bump into interesting scenarios while you traverse the map.
So even if you cleared out a map of all enemies, there's a chance that you'll meet a random stranger needing rescue...
I think that idea would make more sense and work better than respawning.
However a special combat zone (arena? testing ground? Larian QA office?) where players could practice their skills against a variety of opponents multiple times (getting experience for the first battles, but not for repeats) could be one way for hack-happy players to notch their swords.