Repeatedly killing groups of rats doesn't make for great gameplay. There are situations where respawning would make sense in the game world (migration or reinforcements from adjoining areas, etc), but in general Divinity games are not meant to have a strong grinding mechanic, especially with the switch to turn based combat.
There are ways level scaling could be implemented to keep opponents from becoming too weak if you go to a lower level area later in the game. There are lots of ways level scaling can be implemented poorly, though.
I'm not sure how you can use realism to argue for respawning (at least in the case of rats), but also advocate opponents become weaker or stronger depending on your own level.
