Piracy

I use web seeds on anything I'm creating a torrent for that is not copyrighted / free to distribute. The web seed is hosted on my server as a permanent reliable source, but the swarm handles the bulk of the bandwidth.

I wouldn't use it for less legitimate purposes because the risk of hosting the content is far too great. There's also no plausible deniability like there is with simply being a client in a swarm. Hard to argue ownership of something when there's a link to something you operate hard coded in a torrent file.

I think they're fine. All a web seed is, is a direct server download to your torrent client as if you clicked "download" in your browser. It causes no damage to the torrent if they go down, although if web seeds are holding illegal content they pose much greater risks to those hosting them.

They're usually used to make the initial seeding go faster, since you can web seed to the crucial first 10 seeders very quickly via instead of piecemeal uploading the torrent to the entire peer cloud parts at a time and potentially never getting the complete torrent out for days and days.