Perhaps not so useful because it would exclude old users with standard keys.
However needing to revoke the old key I mined one, just in case :)
PoW for events seems for fleasible.
Discussion
I see both as useful.
The way #[10] is doing it where it limits who can post to his relay makes sense to me, it's expensive for someone to keep spinning up new pow32 accounts everytime they're blocked for spamming so it means I can place a bit more trust on that relay not to have spammers on it.
People that don't have PoW keys can still use other relays and retrieve posts from that relay.
Only users with time and, most importantly, the machinery and know-how will be able to access such pow-limited relays. I donāt think you will have spam problems as the number of users in these relays will be quite small in the first place (having in mind the ānormieā population). But again, relays are free to decide whoās allowed to use their services anyway; they can whitelist, blacklist, charge usage fees and so on