There are two aspects of this issue that are usually neglected:
1) The psychological (and possible physiological) effects of a bumpier ride on power output. Somewhere, years ago, I saw a study on this. If your teeth are rattling from the bumps, your power output drops.... This bolsters the case for lower pressures.
2) The loss to "torsional resistance" (I'm not sure if that's the right term) on the drive wheel. This is probably not an issue for most triathletes/tters, who are typically riding in fast, high inertia, circumstances. But when you go uphill, things change....
The uneven force of pedaling up a steep hill gets translated more efficiently to forward motion if the tire is less pliant. In the hillclimb Crr tests I did years ago (5-10mph), it seemed like no pressure was too high. I got very good results in Mtb hillclimb races with tires inflated to 70-80 psi (3X what everyone else was using).
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