Because I believe these oversized host lists are one of the causes. Because I think that the percentage of hosts accepting incoming connections versus the number of all hosts on the net won't change very soon.
And because I think, as this method differs between connection requests from Clientnodes and those from other Supernodes, it could be a very useful addition to the Superpeer concept.
(Clientnodes don't need information about nodes which will only accept connections from other Supernodes)

And about NAT/Gnutella proxy: If we really get this Superpeer architecture to life, we'll be fine without the ability to connect to those hosts which are behind a firewall/IP-Masquerading router.

Guido