Where in my statement do I imply that a wizard does both? I simply state that I have read books and seen movies where wizards/magic users use mind magic in addition to casting other magic. Fireball has been one of the rarest spells I have ever came across in other fantasy books and even though Gandalf casts one in the books it was left out of the Lord of the Rings movies.
The reason you see things like this is because a character that inevitably wins simply because the author says he can is incredibly dull. Magic is untethered from any non-arbitrary causality, and arbitrary causality can take tension only so far.
As for why you never see them combined in literary works I'd say it has more to do with the nature of the magic compared to personality than the lack of ability to do both or avoiding "magical omnicompetence" as you put it.
So a literary wizard who fails to demonstrate competence with Magic Type X is actually capable of Type X, he just doesn't feel like using it. Or alternately, he's capable of Type X, but he's never in a situation where Type X would be more useful than what he can already do. I really can't buy this analysis. It requires either a wizard so pigheaded that he'll use a hammer to turn a screw rather than reach for a screwdriver, or else one who miraculously never faces conflict for which his established shtick is not perfectly suited.
4e doesn't let the wizard do a lot of things he used to be able to do. Even with refluffing, some abilities that were bread-and-butter in earlier editions are simply gone- mind control, instant long-range teleportation, cheap scrying, instakill spells, polymorphs, and long-duration buffs, for a few. Some of these may reappear with specialist classes, and some may be gone forever. If none of them come back, I really won't shed any tears. If I want to play a god among men with unstoppable sorcerous powers, I'll reach for Ars Magica, which does a vastly better job of handling a game where Wizards Are Better. The fact that these powers are gone from my D&D is not a bug to me, it's a feature.
I'm sure some people will earnestly insist that they could have been kept if only WotC had been smarter about implementing them. That's nice, but it's possible to insist that a circle could have four sides if only you're smart enough about squaring it. It's the sort of thing that I'll believe when I see it.
