There are probably a ton of other use cases, like internal relays, temporary relays for conferences and closed events, more utility use cases like I don't know you publish something there and the relay prints your message and mails it to you (silly example, this doesn't even make sense as a feed).

I'm trying to have more crazy ideas, but I bet other people will have them better once the possibility is unlocked in their minds. But also only in the scope of the relay classes listed above we already have enough room to fulfill many more interesting use cases.

Just communities and curation are already enough to justify having relay-feed-capable clients. The curation case is infinitely useful. Almost every one of the "whatever-on-Nostr" use cases (music on Nostr, business listings on Nostr, geocaching on Nostr, uber on Nostr, wikipedia on Nostr, livestreaming on Nostr etc) would benefit drastically from having curated relays, and it could become a profitable thing eventually, as I probably wrote on nostr:naddr1qqyrze35vscrzvfcqyghwumn8ghj7enfv96x5ctx9e3k7mgzyqalp33lewf5vdq847t6te0wvnags0gs0mu72kz8938tn24wlfze6qcyqqq823c0z85e2 (but I didn't reread). There is also the case for "publications"/magazines which I've seen many people talking about and to me that is a textbook example of a curated relay.

Many of the DVM use cases could also be reframed as relays with special functionality and they would work much better and be simpler.