As result of issue discussed in MDL-12579, I filed bug MDL-12603 asking to remove the registerauth option (self-registration) from the "common settings" in the authentication configuration since it is apparently applicable only to a single plugin (mail-based self-registration). However, a better solution would be to make this option applicable to all or most authentication modes. I have specifically a situation where we want to use the POP3/IMAP authentication but do not want to allow anyone to register themselves. Having a restricted-access Moodle server must not force us to write custom plugins or use only manual-only registration, which appear as the only possible solutions.