The questions you ask Owen are obviously answerable by anyone with access to a BGP routing table dump (which is pretty much anyone!).
BGP is many things - it is a topology maintenance protocol, but its a traffic engineering protocol and an attack mitigation protocol. In the latter two cases advertising more specifics play a crucial role. The pressure to slice and dice in IPv4 is a mix of reachability in a space where address availability is under acute pressure, and TE and DOS mitigation. The pressures on IPv6 are predominately from the latter two categories. I suspect that as IPv6 becomes a larger part of the traffic mix (and inexorably that appears to be happening) then the TE and DOS issues become more of an operational concern, hence rising more specifics in IPv6.
owendelong@PK9XYF9GRK-1346 ~ % host www.amazon.com
www.amazon.com is an alias for tp.47cf2c8c9-frontier.amazon.com.
tp.47cf2c8c9-frontier.amazon.com is an alias for d3ag4hukkh62yn.cloudfront.net.
d3ag4hukkh62yn.cloudfront.net has address 108.138.248.64
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:fe00:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:2400:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:5e00:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:7600:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:7800:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:ae00:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:b600:7:49a5:5fd2:8621
d3ag4hukkh62yn.cloudfront.net has IPv6 address 2600:9000:24bb:f000:7:49a5:5fd2:8621