This is what I've taken out of the LimeWire source code, which is unlike the source of other clients, open for everyone. (I'm not a programmer, so correct me if I'm wrong).

If all LimeWire clients are clustered together, it's easier for the leaf nodes to find new Ultrapeers in case a connection breaks. LimeWire Ultrapeers do not forward all pings to their leaves, but only pings that were marked as coming from an ultrapeer. When the Ultrapeers are grouped, it's logical, that each leaf node will receive many more ultrapeer pings.

In order for LimeWire leaf nodes to stay connected, it is better if the Ultrapeers are grouped, so the leaf nodes won't have to connect to normal hosts which would result in increased traffic for the leaf nodes.