Mannahnin
Scion of Murgen (He/Him)
IIRC the opening sequence of Gary's second Gord the Rogue novel involves a mass battle & siege on a Scarlet Brotherhood citadel, and big spells are getting flung around all over the place, demons summoned, etc.The existence of spellcasters has to be really examined in D&D, if you want a game that also includes army and sieges.
I would say that in GH, the presence of spellcasters, especially higher level ones, was especially rare, and they were unlikely to be involved in those types of conflicts. Which reflects his unease.
IIRC, in the leaders of Greyhawk, there weren't many spellcasters, but the highest levels were an 11th level magic user and a multiclassed (??!!!?) illusionist of 15th level. There was also a 19th level cleric, but screw Veluna.
Later in the book Melf ambushes a large bandit convoy all by himself (or with a couple of buddies?) using mostly M-U spells.