I personally favour the passport/OpenID idea, from a user experience point of view.
In contrast to the problem you've stated, if I were to exclusively use my Google account to log into websites, it becomes a single point of failure if the service was down, and if it were to be compromised.
In contrast to the problem you've stated, if I were to exclusively use my Google account to log into websites, it becomes a single point of failure if the service was down, and if it were to be compromised.