Forrest,
This method would have (at least) the following advantages (or disadvantages, from your particular viewpoint):
1) You could reasonably assure that the number of prefixes in an /8 would match what was allocated.
2) Because of 1, if you get the registries to set their allocation policies such that no more than 1024 (or the target number) blocks are allocated per /8, you can guarantee that the number of routes in an /8 is not too far out of wack with the target.
3) You can give those people moving providers a grace period to renumber, say 30 days. Essentially, the time given to clean up the routing tables. This would be a side effect of the "you have 30 days to fix the routing tables or else".
4) You eliminate the wasted space of addresses with prefixes longer than /18 being allocated.
The only problem this leaves is how to decide who gets an /18...
That is a *very good question*. Different answers to this question have *quite different* implications on the address space utilization. Yakov.