Because it spoils the 'reputation' of the original work? If the movie is done crappyly, then people will assume that the anime/manga will also be crappy.
>I'll also never understand the fascination with subtitles and the Japanese voice, which anime fans seem to think is more dramatic.
It's not that anime watchers hate english dubs. It's just that they hate crappy dubs. I watch a ton of anime and I usually prefer japanese with english subtitles, but sometimes the english dubs are better than the original and in that time, I'll stick with the english dub.
It's just a matter of which version has better quality voice actors and dialogues.