I'd support the creation of a true arcane half caster if you could explain how it is a different class from a wizard/fighter multi class with custom subclasses. Like unique class abilities or tying them into the world like the other classes.
I feel like you could create this pretty well with an eldritch knight just by ignoring or changing the spell school limitations, couldn't you? There aren't any major balance considerations; the designers have already said those school limitations are for flavor, and for the most part a 3rd-level spell is a 3rd-level spell.
Curious, do you think would be imbalanced for a wizard to pick any spell from any class in the game?
How's a paladin a different class than a fighter/cleric? How's a ranger any different than a fighter/druid? The concepts provided by multiclassing overlap quite a bit with the current half caster classes IMO. Basically all you are actually doing is making a Fighter/Wizard that primarily uses spells to enhance their fighting capability instead of fighting some rounds and casting spells other rounds.
1) I'm not saying the eldritch knight should be able to pick from any class. They'd still be limited to wizard spells, just not the two schools. (Which, I should point out, they already aren't 100% limited to; they can pick exceptions at several levels even as it is.) And again, the fact that this wouldn't imbalance them isn't my position, it's the developers'.
2) Just throwing it wide open might be unbalanced, simply by virtue of unanticipated combinations, but making specific trades or exceptions would not be.
The really short answer is tradition. The slightly longer short answer is that the paladin and the ranger draw upon the concepts of the chivalrous knight in shining armor and the resourceful woodsman. They're their own archetype in our minds.
The "magic knight" isn't quite the same. We haven't had decades of gaming to give us a more unified idea of what this is.
From a game perspective, the cleric and druid both have features that don't fit on the paladin and ranger. But, 3E did have variant bard, paladin, and rangers as 15 level PrCs, and I really liked them for a while.
At the moment, for me, the "magic knight" does not feel like a fully fleshed out class in my mind. I can't envision enough variety within said characters, aside from spell selection, to fit them in my mind as a class and not a multi class or a subclass of another.
I'm open to being surprised. The Warlock (one who makes a pact with a non-deific entity for power) has become big enough to feel distinct from a cleric or sorcerer. The Warlord, on the other hand, still feels like it could be handled with Bard or Fighter subclasses to me.
What concept does the Swordmage draw upon? I'm eager to hear other's thoughts because I'm struggling to find my own here.