Thank you everyone for the feedback. It confirms my own reasoning. (Who trusts google results these days?)
Question: The flip side to my initial question is, is there a way to allow selective local accounts? In other words, allow accounts created in our discourse for say admins, but require all others to be via github, etc? Can this be accomplished by giving invitations to a prospective admin, and invitations would be the only allowed method to create a local account?