One use case I still am hesitant about for email-as-username is the situation where a family shares an email address for a household. I know that ISPs like Comcast still just provide a main email address when they set up the account, and I expect many people don't bother to make more for the rest of the family.

With all the free email options available I doubt this would be a serious problem. I also know some sites are requiring unique email addresses in their DB now. I simply wonder how many users still do this and if it would negatively impact them.