On a fundamental level, it's absurd to use the same datum as a unique identifier and an identity proof. The best analogy I can think of is signing up for websites with only a password, and then having to provide that password as an identifier any time you need support.
Oh, and the password is almost impossible to change, and you're required to use it to sign in on a host of other sites.
Oh, and the password is almost impossible to change, and you're required to use it to sign in on a host of other sites.