When I speak of origionality I'm not talking about it being closest to the origional source of content, I'm talking about the origional language. And unless I'm wrong, most manga dialogue isn't changed that drastically from manga => anime transitions.
It's the language translation where meaning is lost, the translation of manga => anime often enough isn't that much of a loss. And even if you did make the English Dub directly from the manga, you'd still have that language barrier. And Japanese isn't a language that is congruent with English, there are a lot of incompatible words and phrases between Japanese and English that convey different meanings.
Though in the end that's not the best reason to avoid dubs, but it is one of them (Then again, subs will have the exact same problem if you think about it. It depends on the quality of the subbing and dubbing group. From what I've heard the dubbing groups try to Americanise the wording of the anime while I know personally sub groups don't tend to do that.)
I'm on my way to watching raw anyway, so this problem won't affect me anywhere near as much in future.