> has trained people to expect that they can be sloppy and inconsistent and enter their email address as 'johndoe' or 'john.doe'

Has it? In my experience, even if people know dots and the part after the + don't matter for their Gmail account (and most don't) they know it doesn't matter to Gmail, a peculiarity they can use to create multiple accounts on a single website without creating new email accounts.

(This is different from case insensitivity, which the majority of popular email providers seem to implement.)