The problem I have is that while there may be a few characters in a dub that are well acted, there are almost always bad voices. I blame the directors as much as or even more than I do the actors.
The main reason why a dub would be better is ENGRISH. It's fine if there's only one or two lines to laugh at (Nadesico, episode 2, I think), but too much is horrible, as in, say, Beck? Also, the "gaijin off the street", such as in the wooden-mecha part of Robot Carnival. I want a mix with the mad scientist from the dub, and the rest in Japanese.
And Dragon Half is good EITHER WAY.