Here's a different common sense way to look at it: maximum light efficacy is reached when 100% of the light footprint is met by leaf canopy.
Any less than 100% coverage and light passes through to the ground (= inefficient), whereas any surplus coverage and we can say energy has been unnecessarily expended building leaves that are not required.
Leaves with direct light exposure receive useful blue/red spectrum plus UVA and UVB, with only some green passing through to lower leaves, ie. unusable.
Presumably some of these leaves will assimilate photosynthate / store energy, but not all, and besides how can we tell which ones? In any case, a healthy functional plant will mobilize sugars to where needed most in the plant, so perhaps it is not important. This is a function of metabolism: to use energy as quickly as it can be created for optimum growth, and I expect we are all aiming for this streamlined process with indoor growing, hydroponics or whatever system where time and cost is an implication.
Thinking about it this way brings me closer to accepting the logic of defoliating - strategically and deliberately - as a practice that can deliver improved yield and/or quality, if one knows what one is doing.