I agree in that I can't see where stamina regenerating in combat is positive in and of itself. I don't, however, see it as a gamebreaker unless it was ridiculously high; in that case it would make most other aspects of the system trivial. My current guess is that stamina regenerates and is relatively easy to heal in the current iteration because (1) stamina gets taken down very fast - e.g. taking down 100 stamina is much, much easier than knocking off 100 HP in an IE game; (2) the 100 stamina you just lost might be relatively easy to recover, but the 50? 30? 20? whatever health you lost at the same time, you are going to have to carry for the rest of the dungeon, and it is significant.
In other words, a dogmatic "stamina regenerates? FUCKKKKK" makes no sense, we have to see how the components work together. Is stamina regeneration something that trivialises other mechanisms (e.g. so that even KOs rarely happen, or stamina healing is way too easy)? Is stamina regeneration a supporting mechanism in order to ensure that stamina is very easily depleted, and to more clearly differentiate between pretty much expendable stamina damage and serious business health damage? And, of course, could the latter positive aspects be implemented without having to resort to stamina regeneration, or would it, for example, be too brutal otherwise?
Without knowing more, what I think I'd like is for each class's basic stamina healing ability to be augmented, if necessary, for balance, rather than have stamina regeneration in battle. Or at least, defer the latter ability into learned passives / actives. Having it as a given doesn't seem necessary.