Negentropy is to sync between clients and relays, not between relays. Even single relay communities need it.
Ohhh I see. I'd always heard negentropy discussed in the context of syncing relays.
Yes, it's for syncing between relays as well as for clients, in fact I only use it for relays. My private relay syncs notes with negentropy from those who I follow that support negentropy syncing and NIP-65.
Hmmm well shouldn't this NIP should be optional then?