EDIT: As ubernostrum points out, Persona is solving a different problem than SRP does. However, one of the reasons different identities (username/password combinations) are encouraged currently is because providers can't be trusted with the secret of your password.
We need to move towards protocols like SRP[0] in general so that no matter where I'm logging in, noöne has my password.
[0]: http://en.wikipedia.org/wiki/Secure_Remote_Password_protocol
EDIT: As ubernostrum points out, Persona is solving a different problem than SRP does. However, one of the reasons different identities (username/password combinations) are encouraged currently is because providers can't be trusted with the secret of your password.