If both of them work that sounds like a great feature for most people.

But why require a username, then?

It does indeed seem a bit strange to require a username when an email-address is enough. However, my point was that even though they use usernames, they still allow you to simply login by using their e-mail address. That's a lot better than sites having a 'lost your username?' link IMHO.

We need the username in order to say who edited what.

How about using the email address instead (and offer a username as a display alias only, if people don't want to have their email address displayed).

Meh -- who would want their email address displayed? That's not a safe default. You'd just be punishing the users who don't manage to find the setting.