It's a win-win from what i see it.
Community gets funding they don't have to pay for another translation and make a little more profit.
Well, sorry but "sensei" just means "doctor", "professor" or "teacher" depending on the context, it's not some mysterious magic title that can't be translated.
The folks watching it with me found the Disney sub confusing when I had no trouble keeping up with the original.
So there's a balancing act, and I'll agree that otakus are too quick to leave stuff untranslated, commercial translators are too agressive at picking a word that doesn't fit.
On the other hand, consider a series in which character A calls character B "B kun" while character C calls character B "B sempai".
Most of the time it doesn't matter enough to try and bother with a good way of translating that, but sometimes it comes up in the plot, perhaps several episodes into the show.
Choosing to leave those untranslated isn't anything new, nor unique to fansubs. The Early '90s AnimeEigo translation of the Aa! Megamisamaa OVA left them untranslated in some cases, with a note on the top of the screen.
Yes, fansubs could use better copy-editing, but so could some professional subs. The biggeset offender is that I have seen several instances of English loan-words left in their rōmaji spelling rather than their proper English spellings in commercial subtitles.
Some of the higher production value translations will go for a very literal subtitle translation and a highly figurative dub translation. An example of this is the "Tenchi Forever" restaurant scene translation where The dub is roughly:
Waiter: "Would you like coffee after your meal?"
Diner: "No"
and the subtitle is Waiter: "Would you like tea with that"
Diner: "Yes"
(That one particularly sticks out in my mind since if you watch with sub+dub at the same time you have someone speaking "no" with a subtitle of "yes")For those not in the know, the difference is incredible. Contrast a DVD release with crappy bitmapped, two-colour subtitles with interlaced video to a file with nice anti-aliased subs, often more contextual information/better translations, and in a small file size (thanks to a modern codec[1]) and a mobile-device-ready format to boot. There really is no contest.
To put it another way, it wouldn't surprise me if many people end up watching fansubbed versions of shows they already own just because it's better and more convenient.
[1]For proof of this just see VLC's release notes for all the improvements we have thanks to anime fans pushing adoption of modern technologies.