Well, that's all subjective then, isn't it? I thought LaBrie's vocals worked great on ToT. All things considered, he's a very well rounded vocalist. Not everyone could bring the powerful grit AND the high, soaring melodic vocals the way he can. And sure, maybe he couldn't do something like, death metal growls, but that stuff doesn't belong in DT anyway.
And I mean, for songs like As I Am, it's not even THAT heavy. It reminds me of Metallica in early 90's. LaBrie's style fits that just fine.