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.