I actually completely agree with your comments:
https://code.djangoproject.com/ticket/19844#comment:2

And I don't see the thorny issues you bring up answered in subsequent comments - how can you sanely use a user model that does not have a unique identifier??

If we do want to relax this constraint (which I'm mildly -1 on) then the "must not" language needs to be made less absolute, and perhaps the docs modified.