Didn't think FFXIII was so bad in the acting department myself, and I am not especially friendly towards that game. There was Vanille of course but that was more due to featuring the worst attempt at an Australian accent I've ever heard from someone who is actually Australian.
But honestly, I don't think hiring dedicated voice actors is necessarily better. There are plenty of bad dubs from the usual JRPG/anime crew (and some fantastic ones as well) and some excellent dubs from actors more experienced with TV and the stage (any British VA'd game for instance; we don't have many dedicated VAs). In the end I think it comes down to the skill of the director more than anything else.
As for NMH, I don't think that really had "lesser" VAs, it shared some with FFXIII even. I mean, Robin Atkin Downes is pretty huge and has been in lots of major stuff, and is just as much a TV/film actor as he is a game one. True, NMH did feature a lot of more unusual performances, but I think that's more due to the direction than it is the actual actors themselves.
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