the largest reason that I don't normally watch dubs is because I tend to watch things that haven't been dubbed yet
For example, I was watching TTGL practically as fast as the fansubbers could pump them out, and having experienced it already, had very little desire to watch it again in English. (Actually, that's a partial lie, I didn't know that it had been dubbed until this thread) Still, TTGL is one of those series whose best punch is the very first viewing, every time after that isn't quite as good.