You almost always want their email address so that you can email them a password reset link in case they forget their password, which is even more important on smaller obscure sites where that's bound to happen.
As a user who doesn't want to provide my personal email to random services, I have a throwaway email on a different provider with a gibberish username. Given that signing up for 99% of services already requires an email account (or a Facebook/Twitter login, which themselves require a verified email address), Persona is no less onerous.
