Good point: I missed this. On tubulars, the whole volume of air is on the outside of the rim, doing a useful job. On clinchers, there is an amount of air between the tire hooks (brake tracks), which does nothing in terms of smoothing your ride or protecting against impacts. Add another green tick to the tubular ledger.

Add to this: when you pressurize a clincher tire, the tire beads push out sideways against the rim. This is why clincher rims have inflation limits (tubulars don't), and why clincher rims have to be beefier than tubular equivalents. So on tubulars you can run a much rider range of pressures; lower due to the inherent resistance against pinch flats, or as high as the tire will take, as the inflation pressure does not stress the tubular rim.