PoW is useless really

but NIP-42 lets you keep a track of a user's level of publication of events, and short lived identities have far lower value so it weakens the value of spamming

if a user is spamming with an npub you can stop them from spamming using that npub, and that will eventually combine with clients all using web of trust to reduce the chances their notes that are from npubs nobody follows will get any visibility and thus kill the spam use case