That sounds like a useful thing to have for organizations that want to keep control over their own identifiers, but not tie themselves too much to the internal Omeka id’s of the items.
It could be somewhat related to Developing Persistent Identifier module for Omeka-S, which (if I understand correctly) would include a way to talk to an external system to register and auto-populate Identifier properties for new items.
I agree that a simpler subset, where Omeka-S invents its own identifiers (according to some scheme like a sequence or a UUID generator, perhaps with a prefix/postfix?) could be useful as well!