The people who read the original manga probably feel a certain way about the anime. The same way that people who read books feel about movie adaptions because they know there will be differences in it. A movie can bring things to a wider audience and then a lot of them may check out the original, which they may or may nor prefer.
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.