I have far less issues with people starting with a class according to their background, than people sticking to that class through their epic journey of life even if there is no reason whatsoever to do so.
Up to level 5 or so, keeping with their NPC class, sure, sure. But the enormous amount of XP required to get to high levels implies the luxury of changing class. If PCs can do it, there's no reason NPCs can't. Because an epic-level NPC must have basically lived the PC life to get there, unless it's the kind of world that goes "YOU HARVESTED 1 LETTUCE HEAD! HURRAY, PLUS 500XP!". By RAW, the only thing I remember awarding XP is killing stuff. I can't find the rule for RP XP, but I seem to recall it's optional, at the DM's discretion.
Funny, I thought we were talking about D&D 3.5? I cited OotS once, and I did so because it was making a gag specifically of the D&D mechanics being referred to, not it's own original mechanics.