A lot of it depends on the anime itself. In some cases it doesn't matter one way or the other. In some cases it can, usually if a character seems to have too high or low a voice. The other reason the subtitles can be helpful is that I can watch the movie muted and not disturb anyone.
One of the other reasons might just be this, there are a lot of fan-sub things out there that aren't available for retail sale in the US so, fan-dubbing isn't as common or likely isn't as good so I just figured that was a big part of it too. So if people see new stuff that isn't done at professional quality they see that the subs tend to be at the very least less screwed up than the dubs, so they figure the subs are better by default.
Though I'll make my own comment that in watching a DvD recently the subtitles and the dub didn't always agree on what was being said so I'll say that maybe people find one more accurate than another.
One of the other reasons might just be this, there are a lot of fan-sub things out there that aren't available for retail sale in the US so, fan-dubbing isn't as common or likely isn't as good so I just figured that was a big part of it too. So if people see new stuff that isn't done at professional quality they see that the subs tend to be at the very least less screwed up than the dubs, so they figure the subs are better by default.
Though I'll make my own comment that in watching a DvD recently the subtitles and the dub didn't always agree on what was being said so I'll say that maybe people find one more accurate than another.