Although your point of focussing on the total volume of calories burnt is a very valid one (especially the point about being able to sustain training for longer periods of time), I have to make a nuance here.
Please note that the calorie count of the PM5 is based on E = ( 4 W + 0.35 t ) / 4.2 [kC] (see Physics of ErgoMeters). Oddly, this disregards both weight of the rowing person and the SPM (a portion of work performed, and accounted for in a constant), as well as his heartrate. So the calories in the logbook are approximations at best, ignoring some physical aspects of increasing pace/SPM, and we don't know if these calories are produced by predominantly burning off fat or carbohydrates.