[Merkin]

One really basic advantage to not levelling is to hold down the strength/numbers of monsters. Outside of the pre-set and boss battles, encounters are based on the highest character level you've got. I find it really usefull to level up most, if not all of my characters at the same time, and especially in the early game, resiting leveling up past 4 or five until you can get to Arnika (and acquire better armor/weapons/spells) can be pretty handy.