I don't have an example for you, but if you hide this and someone tries to sign up using the same username for an account that already exists they will be unable to register and either have to select a different email address, or login. We have some customers who have multiple accounts for legitimate reasons, so we wouldn't want to restrict this.
A feature request has been made for a setting to force email address as username, so it's something we're considering implementing system wide.
The approach I would take is assign a class to the div for the form element, and in CSS set it to display: none;