For me, it's a flavor thing. I'm under no illusions that any real person could do what a 20th level fighter could. But, when I play a sword-slinger, I want it to feel like a sword-slinger, not a spell-slinger. A fighter 20 should be a fighter++, not a fighter/wizard. The moves may be exaggerated, but they should still be recognizable as extensions of what he was doing at 1st level.
That means that the +6d6 damage that penetrates DR is fine, but +2d6 damage from summoned flame on the sword isn't. It's not a balance issue. The mega-strike damage is just a much more in-flavor with a fighter than is the flame-strike damage. Making a strong guy, inhumanly strong works, too. Making an improbably jump is one thing, but the fairy-prancing in Crouching Tiger-like movies isn't the image I want coupled with my dwarves.
I guess, if I could pick a mental image for high-level martial characters, it would be "300". I may be missing something, but I don't recall any flaming weapons or fairy-prancing. There were a lot of extremely unlikely defenses, attacks, and feats of endurance, though. That's the feel I want from my epic fighters.
I'm okay with a ninja-like class (i.e. swordsage) or a crusader who channels faith into martial prowess. I just also want the option of having my Leonidas as formidable.
As an aside, I really hope the ranger ends up a martial class. Removing the ranger spells and replacing them with some appropriately extraordinary and/or supernatural (to use the 3E classification) maneuvers that replicate them would be much better, flavor-wise. I'm thinking Shadow Hand with nature instead of shadows. That might even get me to buy into a ranger with d8 hit dice and/or medium BAB (I'd still prefer d10 HD, though).