It is a rationalization often floated for making the fighter strictly inferior in terms of versatility and depth of play (along with TRADITION! of course). The barely-unstated assumption is that anyone who would be interested in playing a fighter must be sporting a lower quantity of grey matter than those wishing to play a healer, or blaster or full caster.
But, the fighter was popular when it was every bit as complex as any caster short of Wizard (and not far behind that) in 4e, and in 3e when it was quite the engaging system-mastery challenge to create a viable fighter build, at all. So, that assumption is unwarranted.
I also see no reason to think there aren't folks frustrated at the complexity of playing a wizard who'd be much happier just blasting away with something like the HotEC Elemental Sorcerer.
And, of course, the Champion remains virtually cognitive-load free, for any who do conform to the stereotype. Maneuvers are a BM feature, and the BM is meant to be there for anyone wanting to play a more complex 4e-style fighter, or an even more complex Warlord, or a 4e-style non-spellcasting ranger...
...one sub-class, 16 maneuvers, standing in for 3 full classes, 24+ builds, and well over a thousand exploits.
But we're worried it's too much cognitive load for the folks what liked having all that?