It's implausible because it's not defined. You haven't told me what these "reserves" are, and so without anything to go on it's more plausible (via association with the fighter's concept as a non-magical character) to presume that they're physical abilities.
What's wrong with them being physical abilities (though, really, there are plenty of non-magical mental abilities, too).
by itself "reserves" doesn't tell me anything. If it's not informing me as to what it is within the context of the game world then it's not associated.
It's not like the word is meaningless. The meaning is quite clear, the hero can make super-human efforts of strength, courage, and so forth, but cannot do so without limit. When he doesn't make such an effort, he is keeping those extraordinary resources 'in reserve.' Thus 'reserves.' It's a perfectly normal, English-language use of the word, not even the crazy jargon that the mechanics side relies upon so heavily.
Usually when one thing tires you out so much that you can't use it again, that level of fatigue affects your performance in other areas, yes.
Well, sure, IRL, /usually/. OTOH, you can be exhausted from doing knowledge work and unable to make progress, but well-able to go outside play a game of basketball.
But, really, why would the limitations of real people, doing ordinary things, have any bearing on what heroic characters from a fantasy story might or might not do?
We are to say, because unlike passive audience members we're the ones who get to peak behind the curtain and see the "how" of things. We get to see what those limitations are, and can determine if they make sense within the context of the game world or not.
That sounds like you're getting meta-gamey, there. If the character knows that if he makes the extraordinary effort to pull off a dramatic stunt, that he won't be able to do it again, as well, right away - and the player knows he has an encounter power and recourse to p42, what's dissociated? I mean, if you're saying the player needs to know /better/ than the character the explanation for the limitation, /that/ sounds like breaking association more than establishing it.
See above. If you're not defining these as magical, then why should they be discrete?
You're begging the question. You claim that 'deep reserves' can't be discrete because they're not magical. Why not? What makes you think all non-magical ability is absolutely generic? In fantasy, magical and non-magical abilities both tend to be represented as being unique skills and talents. A great swordsman isn't automatically also a great poet. A wizard who can conjure lightning can't necessarily make it rain. If anything, magic is more often depicted as a generic resource.
Magic doesn't need to rationalize itself; how magic works is inherently self-explanatory.
How magic works is generally /not explained at all/. If not explaining why something works a certain way, only that it does work that way, is not
No, they're more reasonable. We know magic is inherently supernatural - we have no idea if that's true for "deep reserves" (though here you seem to be implying that they are, since you said that fighter abilities are "preternatural").
We know that, in 4e, per the PH1, that martial exploits are /not/ supernatural, but can perform superhuman feats. Preternatural sums that up neatly. So, the 'deep reserves' that make martial dailies associative, are not magic and not supernatural, but they are clearly extraordinary - not something just everyone has, for instance. That's seems perfectly reasonable, plausible and consistent with such abilities being possessed by heroic figures in a fantasy setting.
See above. Because magic has no inherently defining characteristics - beyond that it's not bound b the conventional understanding of the way things work - it gets to set its own rules.
What do you mean by a 'conventional understanding of the way things work?' The way things are conventionally understood to work in the fantasy genre, for instance, includes magic - and includes heroes who don't use magic, yet perform all sorts of super-human feats, and would be fairly well-defined by the set of common genre memes and tropes. If you mean that magic isn't bound by the understanding of people who don't understand magic - well, sure, that's a given.
And if magic has no defining characteristics, then it explains nothing (as in the case of the English language idiom 'like magic'), and is neither reasonable nor plausible nor understandable by the character, and thus /always/ dissociative at the level of rigour you're demanding of the explanation of martial dailies, above.