The real security benefit, I guess, is that people don't need to remember any passwords. The biggest weakness of password schemes is that most people choose weak passwords that they can remember, rather than strong passwords. If you could consistently derive a password from biometric data, then that sidesteps the entire issue (there are no "weak fingerprints").

And if you could consistently derive the key from the biometric data entirely on the client, then you wouldn't even need an authentication provider, instead transparently treating the resulting authentication token as a password.