Sorting through spells for the Complete Illusionist makes one think about the different magic-user subclasses, and the real differences between them. Is Divination really a school on par with Evocation? Can enchanters teleport? Should they? Why do necromancers have fear spells? Does it make more sense for classes and spell schools to be cause- or effect-driven?
This matters, and it matters in OSR games, because once one moves beyond the core four (or three, or possibly two) classes, one starts looking at archetypes. 3e and Pathfinder both have the mechanical depth and complexity to create classes and concepts around pretty esoteric and narrow ideas, like the fighter that knocks things down, or the fighter that uses a two-handed sword instead of a long sword. OSR games don't have that luxury. Subclasses in the OSR realm embody broad archetypes and concepts; characters that many people on the street would recognize, and anyone with a smattering of fantasy reading or video-game consumption.
Illusionists manipulate minds and energies to deceive people. It's an effect-driven class. Luminous spells shade into serious energy manipulation, while shadow magic gets extra-planar and interdimensional, phantasms merge into charms, and glamers into enchantments. That's just how it is.
Enchanters change the properties of a person or object. In many ways, enchantments are illusions made real. On the mental side, charms change a person's mental state, compulsions enforce a course of action, and morale/emotion spells change an emotional state. On the physical side, enchantments can cause transformations to a physical object's properties and manipulations move the object. (Enchanters largely subsume the Alteration/Transformation school).
Necromancers are a subset of enchanters. They use fear spells, animate dead bodies, and so forth.
Elementalists make the ka-boom. Earth, Fire, Water, Air; earthquakes, fireballs, lower water (wait, what?), lightning bolt....
Summoners...summon things. In many ways this is potentially the narrowest of the subsets, and I'm tempted to leave it out altogether. Illusionists summon shadow creatures, necromancers summon undead, elementalists summon elemental creatures...if it's that easy to give away the summoner's stuff, then that's probably the thing to do.
Wizards get what's left over, which isn't insignificant. A wizard controls magic: Dispel Magic is probably the iconic wizard spell. And Magic Missile, since they get the esoteric energies. A lot of abjuration/protection spells would fall to the wizard. And, since wizards know many strange things, they get a goodly host of divinations to top things off.
The Magic-User, then, is the ur-class, the generalist, the iconic icon.