When the simple_password_check plugin is active and you create a new user that is authenticated through pam, password verification fails. In a way this makes sense, as a pam authenticated user has a zero-length password.
Using the pam plugin doesn't mean all users are pam authenticated (root for example should, imo, never be externally authenticated). For these users it would still make sense to be able to prohibit weak passwords.
I would thus like to request the password check to add support for externally authenticated users so both plugins can be used at the same time.