I don't know, I think the fact that they're used time and time again could be a negative - either they'd have to change their voices and sound more cartoony because of it, or they'd sound exactly like another character, which could be distracting.
Besides, the most important traits for KH leads are unusual enough for animated characters that the experience effect is likely limited. It's easy to imagine that, aside from unfamiliarity with a voice booth, a live-action actor might actually be more comfortable with the level of emotional vulnerability that practically every main character in the series has.
Non-VAs are always going to sound less enthusiastic, I'd imagine, but that's probably got more to do with the fact that we're used to voice acting being somewhat exaggerated in the first place than a lack of enthusiasm on the part of the VA.