Client SSL certificates for personalized accounting
Enforcing of strong crypto : by the tor protocol specs a cipherset is negotiated, and can be not a stron one. But regardless of the transporting onion crypto - a strong 256-bit AES can be enforced during HTTPS handshake
Server authenticity verification: if the Tor HS keys are hijacked, it does not mean that the HTTPS keys are stolen too. Even if they are on the same server.