Just to chime in...
As @eobox91103 said, if you are fortunate enough to join a swarm before the torrent gets deleted and there are still others in the swarm then by all means keep the torrent running in your client. When the torrent gets deleted from the tracker it seems that anyone already in the swarm can continue to both upload and download. As such, as long as there is a full copy available, either from seeders or from enough leechers, it is still possible for the download to finish. I've managed to complete a few downloads in the past after they became unregistered. If you are lucky enough to have the full copy then please do keep it running as long as there are any leechers in the swarm. You won't get credited for the upload on the site, but you will save people from having wasted a lot of ratio on a download they might never have been able to finish.