It's so easy to go and get a throw-away account, which means that email is not best suited for identity as the other solutions.
How is that different from OpenID? Creating an account on e.g. MyOpenID is easy enough - you just need to write an username and a password; even the email field is optional.
How is that different from OpenID? Creating an account on e.g. MyOpenID is easy enough - you just need to write an username and a password; even the email field is optional.