Tony is right -- any registration process costs resources.
agreed, though the cost of registering a domain name should serve as a useful
upper bound. at least with address blocks you don't have to worry about I18N,
trademark infringement, etc.
But, if these addresses are assumed to be not routable, then there
shouldn't be any routing table bloat. Put differently, once can
conceive of three ways to get addresses:
* From an RIR, as PI space
* From an ISP, as PD CIDR space.
* From some other process, as long-prefix, almost
certainly unroutable, isolated space.
actually it's highly desirable if such addresses *are* routable by private
agreement, just not by default.
I don't see why we shouldn't be able to choose from the above three options.