Ask HN: Why does Quora require Username/Pass after a FB Connect Signup?

I'm working on a new web app and aiming to make the sign-in/sign-up as seamless as possible. I'm constantly stumped that sites like Quora, Plancast, etc that offer FB Connect as your primary account all require you to basically create an account afterwards.

It seems that the time saved is all lost by the time you've completed the sign-up process.

Is there a technical or business reason to require users to enter an e-mail/password after signing up with FB Connect? Or is it just a failsafe in case Facebook goes tyrannical one day?