Can't this be used to ensure you're communicating with who you think you are? Either in a TOFU (trust on first use) approach like SSH fingerprints are in practice, or with external verification like SSH fingerprints can be in theory.

The .onion name can't exist without having the private key for it, that's kind of the point.

There is already a private key needed to prove that who you're talking to is the right person: otherwise the request can't be routed to it. That's pretty fundamental to how Tor hidden services work actually.