I'm against it. It adds flavor, I wouldn't want ADOM to turn into one of those scaling games that are so popular these days (elder scrolls after morrowind, anybody?).
If the game was restricted in the proposed manner, I'd have no probs getting my wiz to the bug temple unobstructed, and only leave to face the wilderness at a convenient level 12-13, with all the quickness talents.
Right now I need to be careful to not get ambushed and overrun by large packs of summonable monsters.
If anything, wilderness encounters should be made more dangerous, with monsters that can kill the PC from a range within 2-3 turns. This would prompt more people to stay in PC and VD for a longer time before considering CoC or even ID.