In addition the charisma score is usable to decide things such as whether a witch capturing a player will turn him into a swine or keep him enchanted as a lover.This is an interesting allusion. Gygax is clearly referencing Homer's Odyssey, where the enchantress Circe uses her magic to turn the companions of Odysseus into pigs and then keeps Odysseus as her lover. He stays on her island for a year, then finally frees himself and sails home to Ithaca. Holmes changed this to a frog, perhaps thinking that The Frog Prince would be more familiar to the young audience of his Basic Set than The Odyssey.
Lately I've been thinking about the idea of various sorts of magic-using types in D&D. Generally, these prompt the objection that a single Sleep spell can beat an entire party of low-level PCs. And while this is very true, they remain a standard part of the encounter tables, and in my opinion provide rich encounter potential. It helps to think of the Sleep spell as similar to the NPC having a powerful weapon and requiring negotiation instead of being dealt with in the "standard" way.
One particular role I like for magic-users is that of a dungeon "neutral." I think these are generally underused: creatures and personalities of the dungeon that aren't necessarily harmful, nor helpful; depending upon the PCs' actions, they can be taken in either direction. Magic-users are great for this, in a whole variety of ways. In the early levels of a dungeon, they can be conduits for spells and items that PCs won't necessarily have access to; a 5th level magic-user in the second dungeon level, for instance, can do all variety of divination that the PCs don't have access to yet. And the referee can make the cost of such assistance whatever they like, allowing them to set up all sorts of missions into the dungeon.
The second role is that of a nemesis. An enemy witch or magic-user is a great foe, in no small part because of the threat of polymorph and charm spells. While I am generally leery of the "boss fight" concept from video games, they do make good leaders of dungeon factions. The built-in fragility of magic-using types in D&D means that they will not be there for long drag-out fights, but rather short, desperate struggles as the PCs try to disable them. In the mean time, their abilities make the PCs' lives very interesting, since their comings and goings might be monitored, illusions cast, or many other magical tricks used to deter them.
The third concept is something I've mentioned before: giving NPC magic-users totally sui generis abilities. Circe, after all, was somewhere between a witch and a minor goddess, depending on what interpretation you choose. They can be the source of magic abilities unseen in the books, or even magical items that are unique without being artifact-level objects. (I'm thinking here of the magic sword forged by a witch from lightning bolts in The King of Elfland's Daughter.)
At heart, this dilemma — lover or pig — is an interesting one to place PCs into. It requires care that it doesn't become a railroading tool, but embracing both possibilities opens wide the options for magic-using NPCs as a major force for the kind of emergent dungeon roleplaying that I find really fascinating.