I'm sorry, but I just don't get it. If the number isn't unique and
can't be used and the network in question can NEVER attach to the
Internet, why bother to get an officially assigned number at all?
That's easy. If you have users to use *assigned* addresses for
isolated networks you can always catch a leak of the bogus routing
information into the connected Internet. Try to do that with random
adresses.
The "isolated" networks very often have some physical connectivity
to the Internet (backdoors, firewall machines etc) and the leaks
aren't rare.
--vadim