It DOES matter if Morpheus users don't share files. Each Ultrapeer can handle only so many leaf nodes. Whether you're a leaf node or an Ultrapeer, you still have a horizon within the net. There are a finite number of nodes within your horizon. The more of those nodes that aren't sharing, the less you're going to see.

Assume for the moment that most of the people using LimeWire understand the point of P2P (i.e. they share), and most of the people using Morpheus really don't get it. Before Morpheus switched to gnutella, you had mostly people who were sharing. You connect to a LimeWire Ultrapeer and most of the other leaf nodes attached to that UP are sharing, and most of the leaf nodes attached to the other UPs are sharing. Your horizon was still the same and you still had the same number of nodes within that horizon (possibly even less given that there were fewer people participating), but almost everyone you could see was sharing something. That translated into lots of successful searches.

Today you've got two million Morpheus users, most of whom, at least for the sake of this argument, are not sharing. Prior to the Morpheus invasion it appears there were an average of 50,000 to 75,000 clients online. Following the invasion the average number of clients shot up to about 250,000. Doing the math, that gives us about 175,000 Morpheus users, or 70% of the total clients. So, take what used to be a target-rich environment within your horizon and replace 65% (assuming at least some of the Morpheus clients are sharing) of the clients with leeches, and that automatically translates into 65% fewer search hits.

Just to be sure I've bludgeoned home the point, let's try an analogy. Suppose you're in an apple orchard with trees planted at regular intervals in all directions. You're securely tethered to a tree in the middle of the orchard. The tether is just long enough that you can reach seven trees away from the one to which you are secured. All the trees around you are healthy and full of fruit. Now, imagine the same scenario, except two-thirds of the trees around you have no fruit at all. I ask you, in which scenario are you most likely to not go hungry?

The problem isn't with stopping the leeches from downloading. The problem is that they're taking up space that used to be occupied by clients who shared.