I think that most probably, the IANA registries will remain empty.
But I think we need an "escape hatch" for the few cases where people
feel really badly about the current state of ISO 639.
This might also be a good tool to make sure that any legitimate needs
are met in 639-bis; an IANA registration would mean that at least one
person who needs a specific language code has chosen to register his
name in an accessible place, and lack of such registration would indicate
that the Internet has no great use for further codes in ISO 639.