How useful is this, though, for just a username?

I would think the purpose here would be that you would use unique aliases per service to limit your own risk in the event of a site breach. However, the vast majority of websites these days require a username and an email address. In which case, if I've got 50 unique usernames but they're all tied to a single email, how much am I really protecting myself if the email address gets included in the breach?