They are downloading and redistributing pirated movies. They're basically helping piracy, and they're an easy target. They'll get shut down eventually if they don't die sooner on their own.
Other types of legal software - I was given the option of using bittorrent when I downloaded humble indie bundle games, blizzard uses BT for world of warcraft patches, some large scientific datasets, large password dictionaries / rainbow tables and a produced for BT TV series are a few of the examples I've seen. But of course it's hard to consider much of this 'substantial' as compared to the traffic volume of pirated content.
There are other examples of services like this though, so who knows.
World of Warcraft
[1] http://theslip.nin.com/ (completely free)
[2] http://ghosts.nin.com/ (order includes a "free" option, but only paid one offers lossless and pro through BitTorrent)
The NIN BitTorrent system includes a token that will allow you to start the download "once" (in fact it seems you can start the download many times during a limited time span, then any started download will be allowed to complete even past that cut-off, but the tracker will refuse new fresh downloads).
The system is strict enough to prevent blatant copy-pasting and email the download link, yet resilient enough to not get in your way.
BT is a data transport medium with perfectly valid use cases. It can very helpfully reduce swarm costs on various intense digital products releases. It's basically bandwidth parallelization and DHT helps to palliate possible tracker downtimes/overloading. It's a shame official release channels don't use it more.