Well, it is is a children's franchise, isn't it?
If the shows were to be realistic, Ash would most definitely be losing left and right, because once he learns the best way to tackle and opponent (and mostly because the Type efficiencies in Pokemon are in a scissors-paper-stone system) the battles are going to get predictable. Very predictable. And we all know what happens when a kids' cartoon gets predictable - the company loses money because no one wants to watch it anymore! The horror!
Unevolved Pokemon are usually there for the cute factor. I think in the first generation of the anime he carried around at least one fully-evolved Pokemon, but in the subsequent generations his Pokemon hardly seemed to evolve. Granted, Pikachu refused to evolve, but the rest who technically should have evolved via leveling hardly do either.
Actually, about Liepard... I find that it's attacks in-game (referring to BW 1 and 2) are pretty weak, so a Krookodile should be able to withstand some hits. Given that both are Dark-type as well, Type resistances should come into play. I think if anyone else were to do this they would most certainly change their active Pokemon instead of just waiting out the potentially long battle. Normal attacks against Rock, Dark attacks against Dark. Whooo, yeah. I think I'm going to just mash the 'A' key while doing the dishes.
Having a different protagonist for each region won't have the same effect as having the same protagonist everywhere, and you do need said protagonist to somehow get around regions after exploring them more or less thoroughly. The formula here would be quite the same as the game: Enter region, beat Gym Leaders, Beat Team Whatever at Whatever they're doing, Meet a Legendary (Okay, so this mostly happens outside of anime now, in the movies instead), crush a few League people, then set sail for the next region. Like in the game itself, but do you see it getting old?
So, verdict. Yes, he is a bordeline Stu, but as I said above, this is a kids' show about a video game. If Ash really were to play as an experienced Trainer, it would get pretty boring quickly, so yeah.
Bleh. Anyway, high five, fellow PokeFan.