Imagine that you are driving down the road. You need to know where you are. You consult your road map.
Suddenly, the guy in the back seat says, "Hey! There's a tree out there, but no tree on the map! What a crappy map!"
You say, "For the map to be representational in the way I need it to be, it doesn't have to mark every tree."
"Oh yeah?" the guy in the back retorts. "You admit that your map does not serve to really represent this area well. We're getting closer to being on the same page."
The simple answer is that the magic systems of fantasy fiction do not have to map onto the D&D magic system with 1:1 correspondence in order for one to represent the other well with respect to its use in the game system.
As has been pointed out already, no fictional universe has a system of magic that, as written, would work for the needs of a role-playing game, unless it was specifically derived from a role-playing game. Nor should a model correspond 1:1 with what it models in all respects, or it ceases to be any easier to use than the original which it seeks to simplify. A map that shows every blade of grass is too unweildy to use as a roadmap; a magic system that tracks every condition in the game world to determine results is to unweildy to use in a game.
Given the needs of the game, the D&D magic system does (IMHO) a very good job of both maintaining a fictional feel and being workable in a game system. Obviously, it is not perfect (or I'd not have houseruled it myself!) and people will have differing opinions on how well it meets each of the goals aforementioned.
That it is not an exact model, especially as one gets more exacting, is not a valid argument IMHO. Certainly no more so than saying that I cannot find a novel in which a longsword does 1d8 hit points of damage, or that I cannot find a novel in which any cleric casts precisely the same spells as in D&D. Certainly, if this is the claim levelled against the current system, the same claim will be equally valid against the per-encounter system of 4e.
And, for the record, wizards in Conan need to rest more than 20 minutes. In the one Conan novel, it is the whole night, as I recall, and an exact number of hours might have been given. In the passage that I previously quoted from The Gods of Mars, John Carter and Tars Tarkas slept fourteen hours.
RC