How would you hack such an email? If the user logs in with username(not public)/password instead of email(public)/password then would be really difficult to hack such an email account... Because you don't know the username. The email has no use to log in, therefore not even brute force is useful.
You brute force the username+password pair. As a bonus, you don't need to know the e-mail address, so you're automatically trying to break in to all the accounts in parallel.
How is that different from having two passwords or a password of double the length? Worse, people may put in the first part of email as username defeating the whole purpose.
