Nice necro, but why not post here again I guess...
I think the action economy advantage that casters can get at mid to high levels really seals the deal, but even before that, spells allow primary casters to function at a whole different level of play. Spells like Teleport, Contact Other Plane, Planar Binding/Ally, but also lowly Rope Trick, Dispel Magic, Fly, or Augury, heck, even Detect Magic (!) see to it that casters have the option to know about and circumvent difficulties beforehand, while a mundane usually has to fall back on skill use. Casters are not just "moar powerfuller", they're more versatile. And that versatility boils down to obtaining a certain amount of strategic control (of the narrative itself), while not sacrificing tactical control (of individual encounters).
This is an issue that I'd argue has always been in the game to a certain degree. However, in pre-3E, Fighters were at least very, very powerful on the battlefield, while Wizards or Clerics needed just the right prep and just the right circumstances to rival them tactically, due to spell disruption and the saving throw mechanic. Casters also had less spells and often much less consumable magic items to increase their versatility tenfold. So pre-3E I'd say casters were more focused on being strategic assets, while Fighters were tactical assets. In 3E (3.0, 3.5, PF...), casters don't give up sheer power anymore.