I'm with you. We're trying to get over most of that in Persona by using email address as identifiers, which most people already view as proxies for identities (home, school, work, etc), but time will tell whether or not that belief pans out.
On the other hand, designing authentication systems seems completely tractable. To that end, Mozilla already relies heavily on Persona, so it will continue to exist and work for authentication even if external traction is slow.
To help people manage identities, the Persona UI does allow you to pre-load a bunch of your addresses, but the connections there are never revealed to outside parties. In the future, that information will be managed completely locally in your browser if it has native support for Persona.
Just imagine you don't have to carry around all your loyalty cards, the tailor at your store immediately knows your size, you fill out a standard form very easily.
It's one of these things IMO that would create much joy and convenience in the world. People however rightfully criticize the possibility of abuse here. Indeed, having ready access to identity is something not easily to digest to many that love the anonymity of the Internet. Many open initiatives like OpenID have not seen the coverage needed to make such a system happen on a broad basis, Facebook Connect is probably the closest solution. Mozilla BrowserID is the next ambitious project that tries to tackle this space, however I question whether one could ever design an identity system, or whether it just "happens" like Facebook and Twitter showed.