There are at least a half dozen competing cashu wallets. Not sure what you mean. Should they all support nostr-backend? Probably yes, but it's a relatively new idea.
Discussion
I mean that apps shouldn't hardcode one in.
What I think is missing right now is the ability to send and recieve cashu tokens over NWC. You should be able to zap cashu tokens directly and when you recieve them, your nostr client should transfer them to your NWC connected cashu wallet without needing to go over lightning.