I let Magic-Users gain bonus spells, by substituting their INT score on Wisdom table II in the AEC.
I've run about 8 sessions so far, and this hasn't wrecked my game. It balances itself out. Sure the den full of goblins or kobolds is going to probably be a cake walk if you can knock them all into dreamland before they have a chance to attack, but even with 3 Sleep Spells, lost initiative can result in a hit spoiling the spell, and one solid hit can put a low level MU down for the count. Remember, there are monsters that are immune to Sleep, so it's not going to do the haughty little wizard any good to have head full of sandman mojo in a dungeon full of zombies. Also, whats good for the PC is good for the NPC. So that wandering monster roll that comes up "NPC party" could very well have a magic-user with a full clip of spells as well.