What is the point of adding fighter levels to a goblin? In classic D&D, all a fighter is is attack plus hit points - just make the goblin higher HD and be done with it!
Likewise adding MU levels - the goblin (or whatever) doesn't need the additional HD and attack bonus. Just give them a few spells and you're good to go!
Thief levels - just give the monster good bonuses in Stealth and Pickpocketing.
Exactly this.
But try making a level 6/6/3 mind-flayer fighter/rogue/assassin or someshuch and the GM is in for a whole world of pain.
It forces the GM's hand as far as what the monster can/can't do, and dick-headed players will call (or try to) call a GM on any little change/tweek once they see it.
You end up with half-dragon ettin barbarians who have spent their entire lives living in a major city. Really? Barbarian? WHY barbarian?!? Oh yeah..."'cause thems tough n' such..durrrr....". *THIS* is why classes-for-monsters pisses off so many players (myself included).
The idea of 'tougher' monsters and stuff is good...but then trying to finagle numbers, stats, requirements, restrictions, etc.?
The problem is how much guidance the game gives you to make a mind-flayer who has some sneaky attacks, and can also wield a mean longsword on the frontlines.Easy solution: Don't make a level 6/6/3 mind-flayer fighter/rogue/assassin just make a level 15 mindflayer assassin.![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.