I would posit that it is not transactions at all that are important, but users. And more to the point, not all users are created equal (not all users equally add to the network's accessibility/robustness) -- and that is the essential flaw in Metcalfe's law. Transactions tell us very little about the nature of users; for all we know, a single entity or group of entities are inflating transaction volume to make the network appear more robust than it really is.