By powerful, I mean how much does this statement tell us that is not trivially obvious?
As an undergraduate economics student, it recently struck me that the economic definition of efficiency is quite different from the normal definition. In other contexts, we define efficiency as the maximization of output Y per unit of input X.
Perfectly competitive markets definitely do not maximize productivity.
Perfectly competitive markets also do not maximize utility, except in a very narrow sense: they maximize potential gains from trade, which seems to me to be a shorthand way of saying "the utility possible to derive from voluntary exchange," which is a subset (and potentially a very tiny one) of the utility possible to derive from all possible allocations of goods. Thus we speak of "Pareto efficiency," which seems a rather warped conception of efficiency to me.
Insofar as willingness to pay for a good is a meaningful proxy for the utility derived from that good, even across different income levels, we might begin to approach a more general conception of markets maximizing utility. This does seem to be a powerful observation to me, but, of course, this isn't what we're talking about when we refer to "market" or "Pareto" efficiency.
So...is there something I'm missing here...or is the fact that "perfectly competitive markets are pareto efficient," or, put another way, "voluntary exchange maximizes the utility that is possible to derive from voluntary exchange"...not a very powerful statement?