OTOH, facebook id != identity, neither is twitter username. There will always be some id that you won't be able to change. But you are right that email is a bad choice, and i was surprised they give away the email address to developers. They should provide the browserid.org ID only.
it's a compromise move-- the reason most sites today rely on email, fb, or twitter is that it's a way to 1) contact the user, and 2) tell/help the user to tell his/her friends about something.
the team is in talks now about cooperation with another mozilla labs project centered around 1-off email generation for quick and easy anonymity with sites
