A user deletes their Facebook or Twitter account. I don't have their email address. They try to login and can't. They have to now email me and prove to me which account is theirs so I can reset the password, give it an email and let them regain access. The account may be a free account but it can still contain data a user wouldn't want to lose. Ensuring from the get go that they can recover their data just makes things easier.

Regardless of how legitimate you think my usage of email addresses is the inability to even ask for it _is_ a downside. For those use cases that do require it immediately you now have a system where the user clicks the 'Login without filling in a form' button and gets thrown to a form anyway, which is exactly what we were trying to avoid.