I think it'd be pretty pointless to dub it to another english dialect. The accents, and dialects gives the audience more feel to the atmosphere, of the place where the movie is set.

For example: Braveheart was done with Scotsmen, englishmen, and Irishmen. Wouldn't it be rather dumb if it were done (or dubbed ) in a North American accent? Mel Gibson did work hard to nail down a Scottish accent after all, (although it wouldn't surprise me if many Scots feel that it sounds fake). The reason Mel did that, is because its better than speaking in his Aussie, or his North American accent. It gave the viewers more atmosphere to the setting.

Likewise with a movie like Tombstone: They did the best western and southerner accents they could. It be really stupid if were done, or dubbed in Australian, or British. It would take away from the setting and atmosphere of the story.

The dialect of English used gives you a sense of the location the story is set in.