The idea of a torrent is that if you're only missing the last 5mb of a 1GB file, that's all that gets downloaded. If you're downloading in sequential order, the last parts of the file are going to be less well-represented than the first parts. Not much of a problem if it's well-seeded, but they're not always well-seeded.

Oh how many times have I tried to download a file where the most complete seeder only has 95% of the download because all the people who had finished dropped off.

To be fair I think this could be made better by integrating the media player buffer with the torrent client. While ever it's able to keep the media client fed it should also try either download random blocks or maybe more intelligently fetch the blocks with the least availability.

For high efficiency the rarest piece that you are missing should be prioritized. Of course it only matters for torrents that doesn't have many seeders. As long as only a minority of peers are streaming the swarm should be able to bear it.

Yes. I'm the original inventor of "swarmstreaming". If done correctly there is actually an increase in efficiency, especially if some of the users aren't watching the whole stream.