You can already automatically block/drop people who don't have enough "friends" in Gnucleus and some other clients, meaning nodes with a limited horizon. That is picking on those poor people who don't have a lot of bandwidth, IE modem users or people in another country.
Those internal private college LANs won't let you in from the outside, they block you too. They also block anyone on the internal LAN that doesn't have the correct LAN name entered.
At 300 plus nodes they don't seem to have a problem sharing at all so it doesn't matter.
Your horizon on Gnutella is limited even now to a few thousand nodes (or less). You are blocked from the rest of the network, how can you move around to other areas?
Blocking is already happening in many ways, this is just the next step in giving the user more choices, and more power over how he shares.
Some people want to create their own semi private network of friends, you can easily do that now and block any "outside" connections.