There are already compelling legal uses for file sharing using BitTorrent; for example: distributing Linux ISO images.
There is another use that makes all the sense in the world to me: I would watch more network TV if I could watch shows when I wanted. Why don't TV networks submit lower resolution copies WITH COMMERCIALS of their shows as torrents?
Clive Thompson makes this great point in his article in WiRED (January 2005): any media with embedded commercials can be distributed cheaply using torrents. Thompson argues, probably correctly, that distributing via torrents does not work for the movie industry. However, how about ramping up product placements and perhaps do toss in a few commercials.
TV and movie distribution would need something like the Neilson Ratings for TV in order to get proper credit from advertisers.
iFilm.com seems to do alright streaming video with commercials before the video clips.
This may not be a popular opinion, but I wish people "illegally" (at least in the US) trading copyrighted materials like MP3 files and bootleg movies would cut it out. I spend just a few dollars a month on the iTunes music store and that gets me more songs to enjoy every month. I would like low cost ways to download and watch movies also. Like Thompson points out, most people do not want to permanently keep video: download and watch once or twice then toss it. This especially works when material is free, paid for by advertisements.