That way I don't have to remember another identifier and we already trust at least part of our identity to our email provider. Not perhaps as open, but much more approachable as a user.
Other examples that all suck for this reason: Apple IDs. Windows Live ID. Jabber.
Erm, to clarify, Jabber isn't an authentication system. It's a decentralized IM network, structured similarly to email. A user is identified by their username on a given host, just like email. It (sensibly, in my opinion) re-uses the same format for that, user@host (I don't think user!host would be quite as intuitive...).
This does not mean a Jabber ID is an email address. It can be, but they are two distinct properties of any given identifier. So saying it 'sucks' because of the format of its identifiers happens to look like an email address, and some services choose to enable both email and IM on the same ID, is stretching it a bit.
Though on many mail providers these days, it's the other way round that's true. A lot of people use GMail, but few realize that this means they also have a JID (Jabber ID) and can use XMPP, since Google opens their servers.
It's a shame, really. Google seriously missed a chance to kill all of the mess of a thousand and one IM providers (each with their own, proprietary protocol) and replace it with the open and partially decentralized XMPP protocol, which anybody can implement and run.
That is. They would then have to ask you permission to spam you.
When online sales first took off, credit card theft was a huge concern. Even though nothing would go wrong for the vast majority of people fear was enough to make users and vendors go to great lengths to protect data. Not a perfect analogy but conceptually similar.
I spent a day implementing openID for the users of my website, because I realized, hey, what a cool idea, a URL can represent a single user on the internet, and that user can authenticate against it universally.
The sad truth was that I could not expect a single one of my users to even understand what the hell was going on, because for most test openID accounts I set up on yahoo, etc, I couldn't figure out how to use them. Only the hand-holdy sites exclusively for openid even bothered to tell me what my personal URL was and how to use it.
That was when I realized that Clickpass only exists because the implementation of OpenID was a total pooch screw.
If the OpenID standard had required it be simple, like the URL must follow this template - google.com/openid/kevinms and yahoo.com/openid/kevinms, and the user just pasted this into the box, I think it might have been a success. But because they didn't, and they convoluted it more with the concept of your "unique identity on the internet", you need third party services, which are unnecessary layers that are completely confusing to the user.