[mdaniel]

My understanding is that mere swarm membership is sufficient, no need to host anything

That's my understanding of why private trackers ban folks who upload private .torrent files to public trackers because the infohash is a rendezvous point of private and public consumers via DHT

[accrual]

It's kind of like walking into a room of people with full or partial copies of a copyrighted pie, but there's one person in the corner (the copyright holder or someone on their behalf) taking notes of everyone who comes and asks for a slice.