Earlier quoted context omitted.
it certainly is a very, very convenient way to create a new account.
convenient? if someone is not already logged in their email, they have to type an address and then login. Is that easier from having a simple username/password signup page?
Also: if you can use email, you can use posterous. This opens it up to a lot more people who are afraid of "setting up" and remembering another password and confusing web interface.