Gandalf might be a particularly bad example, since Gandalf is mostly a scholar-archtype with a little magic equipment/abilities and quite a bit of heartiness. You could probably model Gandalf in D&D as a rogue or a bard fairly well.

Then again, perhaps Gandalf is a good example of what generally comes to mind with the "wizard" destription. When I think about wizards, I think about a character with considerable arcane and obscure knowledge who puts that to use to apply magic to situations. That is, while the party ranger might know the herbs and treatment (or "spell") to remove poison from a wound and while the party rogue might know the proper mixture of powders to make them seem invisible, the party wizard is specialized in knowing these things and so can create all that and more from the knowledge they've acquired.

Mages with a big bucket of MP and a large spell list are sort of an abstraction of that concept: rather than forcing the player to collect specific item lists or the DM to moderate what components are available when, the characters just have MP pools to make everything more simple. Mages as magi-archers seems kind of backwards to me, starting with the idea of "Mages have to be characters with large amount of MP and lots of spells" and then trying to balance it somehow from there. It is also kind of sad, because it pretty much eliminated the potential magi-archer character, which could otherwise be a cool idea.