Hiya.
IMHO, monsters with classes was/is a bad idea. It has three major problems.
1) Prep time, as mentioned, increases drastically. "Weak" and "simple" monsters, no problem...like, say, a 3rd level orc fighter. 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. Don't even mention magic-using classes...ugh!
2) If 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. "That orc is a 3rd level fighter? And he has whirlwind attack? I call schinannigans! He can't have that feat at level 3 and does he really have a 13+ Int? Cheater GM!"
So, basically, because the GM decided to slap 3rd level fighter in front of the orc, the GM is actually *limiting* what he 'can' do with that opponent. Extend to it's logical conclusion...that a GM will just put in all the requirements as needed (re: he's now a 5th level orc fighter with a 13 Int and 13 Dex), and you have the makings of a true migrain for something that *should* be as simple as the GM thinking
Hmmm...I'm going to give this orc whirlwind attack and an orc double-axe...that'd make him a tough warlord for these guys...should be quite the fun challenge!
3) In relation to #2, the class-for-monsters thing has spawned one of the most hated things (for me anyway) I've ever seen; GM's with no in-game logical thinking capability. Basically, 3.x GM's that I've had the (dis)pleasure of playing under or talking to have all more or less put levels on monsters to make the monsters "tougher"...in spite of it makeing absolutely
*no F'ing sense at ALL*. This has, unfortunetly, been consistent even with so-called 'profesional' writers. 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). I don't mind fighting an extra-tough ettin. But then having it 'rage' on me when I know it's basically been living as a guard for a bad guy in a major city it's whole life...grrrr! At that point, there isn't any reason to even attempt to try and think or prepare for "likly challenges" becasue there is no longer any such thing as 'likely'. If there was no such thing as "classes for monsters", I honestly wouldnt' have ANY problem with it. Why? Because I know the GM wasn't using a 'class' for the monster; he was just giving it a berzerking-rage type ability. The difference may be subtle, but it's there. (yeah, I'm a picky old fart!
).
So...'classes' for monsters was/is a very bad idea. The idea of 'tougher' monsters and stuff is good...but then trying to finagle numbers, stats, requirements, restrictions, etc.? That's insanity and contrary to encouraging GM's (and players) to use their brains. All IMHO, of course.
^_^
Paul L. Ming