I've seen at least one case where a customer has experienced user logins, and therefore automatic user creation, while the system is undergoing its initial site import. This of course interferes with loading the import, causing unique key constraint violations when the imported user id clashes with those created by the logins.

I would think that all users should be barred from logging in via any method, including SSO, while a site import is taking place, since anything they do during the import may be destroyed anyway. This is certainly the case for initial imports. I can imagine that during a restore from backup after a minor data corruption there may be cause to allow users to view pages.