-Evil wizards come up a lot.
-Writing out which spells of which level they have is a pain in the ass, especially because they might die in a round.
-Always giving all of them infinite spells of the appropriate level removes an important tactical limit that makes wizard fights interesting.
So GMs might be interested to giving NPC wizards a limit on what they can cast, but one that's easier to work with than the one PC wizards have.
Here's one False Patrick and me are using in the upcoming (really upcoming: it's in layout) Maze of the Blue Medusa book. It's based on the following additional observations:
-You're going to be keeping track of the wizard's hit points.
-Even if the spell selection is written out for you like in a published module, you still are always going to have the player's handbook and/or its spell list and descriptions immediately to hand just in order to run the game normally.
A most excellent Librarian, 12' tall. Guards the SEEPING CHIMES from interference and knows what they are for.
Atk d12 bite or by spell
Can be harmed only by magical weapons--except fire, which does doubles damage.
Spells: Gruel can cast any magic-user spell of levels 1-4 at a hit point cost to herself equal to thrice the spell's level--so Magic Missile would cost her 3 hit points. The cost of healing spells is deducted after the spell takes effect.
800 gp in ancient bracelets.
Notes on this:
-Obviously you can adjust the hit point cost for different kinds of wizards to like 2/level or 10/level or whatever and for different editions. This particular version of this monster was designed for Basic-style monsters, who have way less hp than 3, 4, and 5 edition monsters.
-Again, this presumes you've got the PHB and its spell list by level right there anyway, so it's no biggie to look down the list and decide what spell you'll use.
-If you're tempted to use this for PCs, there are hitches that take hold: outside combat, it gives them them infinite healing (if they have any healing spells) which in turn gives them infinite spells, which in turn means infinite growth, shrinking etc etc. It's good for combat only.
-Players playing hardcore system-mastery-as-tactic will be frustrated by this, but, really, let them be. It violates the system's rules but not the genre's.