Regretfully, noone has come up with any sort of economic (the only one which works...) dis-incentive countering such behaviour, so we'll end up by muddling along.
BTW, this argument is address-family independent...
Indeed. But, the reason why such non-ISP LIRs might become more prevalent nowadays is IPv4 depletion. We already know that IPv4 isn't going to be sustainable though, so I don't think it is anything we need to worry about or attempt to "fix" or "prevent" through implementing restrictive policies.
True, but that doesn't mean that everyone needs to think this is just A-OK. I'm doing my bit by frowning my nose and muttering here (for all the good that will do).
In the longer term, in the IPv6 world, such non-ISP LIRs will again be just a corner case that exists in limited numbers, and probably only where there's actually a good reason for doing it that way. Allowing for them to exist thus won't cause significant harm to the routing table.
Maybe for the IPv6 allocations, yes, but at present IPv4 and IPv6 is served by the same network infrastructure, so whatever silliness happens in the IPv4 world will affect the infrastructure we use for IPv6 as well. (Yes, that's another argument than what I started with above.) Regards, - Håvard