Don't like getting hit up for an email address right off the bat, but Marco has a pretty solid track record.
It would also be nice to have an explanation as to why it's needed (web client etc).
> Why an email address instead of a username?
> Password resets.
> I tried usernames with Instapaper at first, but people forgot their passwords and got locked out of their accounts all the time. It was by far the top support problem. Email addresses fix that. (Nothing’s stopping you from entering a fake address, but if you forget your password, I won’t be able to help.)