I guess some of us just don't really seem to understand why Martial Dailies are unique in requiring an explanation, and why daily powers never seemed to bother people until 4E. I think it stirs controversy among 4E supporters because of how frequently *some* of the 4E critics bring in emotionally charged arguments that often mischaracterize Martial Powers into being "spells for melee classes" and Second Wind as being "a heal spell for every class".
The arguments are generally misleading and seem to coming from an established bias against 4E that discounts reasonable fluff explanations as being unacceptable. In general, most of the criticisms are disingenuous, and seem to be more of justifications for not liking a new systems, since other equally arbitrary mechanics have always existed without such a furor.
I NEVER heard people complain that "once per day" powers in 3E effectively made other classes into casters!
For some reason, some people can accept the limits of spell casting due to mental fatigue, mental stress, focus, and the level of mental discipline and arcane knowledge. Apparently going from Arcane to Divine doesn't seem to be much of a stretch. When it turns into a limit in how many days a bard can do his bard perform power, nobody questions the per day basis. I never heard anyone ask "why can't my barbarian rage more often", or "why can my Rogue only do his Defensive Roll once a day?"
So, how do you explain Martial Daily Powers? It's a doomed debate because it's really an arbitrary dislike. People can accept Vancian and even 4E style powers when they are magical, but not martial. It's an arbitrary distinction, that is just as arbitrary as tons of other rules that are in 4E, and in every previous edition of D&D ever made.
Personally, I don't see why it can't merely be just the physical counterpart of the fatigue, stress, discipline, experience, focus, and knowledge required for other disciplines with other power sources to perform their abilities.
Maybe you need better fluff than that, but personally I prefer to explain an interesting combat maneuver to explaining why a seasoned and experienced war veteran just stands still and does round after round of full attacks (or even repeatedly performs some combat shtick over and over again that he built his character for).