I happen to be quoting pemerton, but this is applicable to several of the recent posts.
I think this is attempting to take the discussion in a straw man direction. The d&d wizard has never really shared anything more than a name and a pointy hat with gandalf. There are middle earth RPGs that attempt to mimic the way magic is used in the books, d&d just doesn't do that.
For an effective discussion why not talk about any of the hundreds of other fantasy magicians in print, almost all of whom would provide a more useful basis of discussion?
Surely that would be worth the effort, eh?
What about Merlin? Any examples about him throwing fireballs or disintegrates?
What about the kind of Wizards Conan fought? Did they disintegrate people much, teleport, fly? And was he unable to survive against one of them without protective spells cast by friendly mages?
The mages in Harry Potter's world seem to be very different from D&D in some ways, though they may seem closest. Yet, the whole way how potions and spells work seems different. And truely deadly spells are disallowed and used basically only by the evil spellcasters.
What other mages are there? I am not much of a fantasy book reader, my perception seems to be mostly colored by more mythological figures than literature.
The Discworld mages seem to have similar powers, but they also seem to be inspired partially by D&D influenced examples (which may be a general issue, distingiusihing D&D inspired material from the rest). And even there, no reason to assume that there fireballs deal more than 3d6 in a world of 1d8 swords, or that they can cast disintegrate. (They can turn people into frogs, though).
Two cents,
Let me remember you that Gandalf wasn´t:
a) Human;
b) Someone who studied magic at some point;
c) Someone that used anything like 'arcane magic', by D&D standards.
He was some kind of divine agent that was sent to Middle Earth to help others. He was much closer to some kind of 'angel' or something like that, by D&D standards, than a wizard (someone with a class, etc.).
And during most of the LotR, he is a plot device. When the group has a problem they can´t handle or they are lost, he shows up.
Stylistically, Gandalf does look nothing like a Angel, though. Angels are these beings with feathery wings.
Or, if you talk in D&D terms, they cast, say Holy Word and several other Clerical spells. Which also looks not like anything Gandalf ever did.
Stylistically, he's a mage. If someone that hasn't read all the background material to the LotR, they will certainly call Gandalf a mage, and if someone would discuss how he imagines a mage to be, he will cite Gandalf.
So, I think, he essentially _is_ a mage in how people envision mages.
But you're certanily right that he doesn't use arcane magic by D&D standards - he doesn't cast fly, disintegrate, fireball or Teleport. But is it that the vision of a mage doesn't match D&D, or that many people simply have false visions of what mages are.