If your client connects to just one peer you essentially gain access to the entire bittorrent DHT network. As a dramatic simplification, your request for the rare torrent would propagate until it found the peers seeding or downloading it.
Although this is quite the extreme case, and not really the purpose for Bittorrent to begin with, I guess. The main interest is load balancing of popular downloads, not finding the one person with the one file for you.
Of course, TPB switching to magnets doesn't really affect this. Torrent files are still used to provide information on the download, they're just distributed from within the P2P network instead of TPB itself. If you have a torrent file to share with an individual or small group instead of the whole world, there's no reason to stop using torrent files directly.