What I don't understand is why this is such a mystery (speaking to the OP question). I mean why would someone say something like this that is very different than what most growers do, without explanation. This makes no sense, that there would not be an explanation so people have to guess and parse.
Stirring the pot so to speak?
There is probably some minuscule difference in growth which would be impossible to document, anyway. If water only reached the photosynthesizing surface via the internal system, then all the water that got to the leaf would have minerals in solution and be concurrently richer and theoretically leaves might grow more, but that assumes that the amount taken up directly via water on leaf surfaces make significant contributions, or worse, -are empirically measurable. Then a question: how much of the uptake of C is provided by C02 from the air which is part of the exchange in transpiration for which H2O is necessary? How to measure that?
What you give up is also probably not worth the candle. Foliage gets dusty, being subject to daily dew that catches dust in the air and if not washed off periodically would filter out some fraction of light. Again, hard to measure, but if you think it's insignificant, stand under a pine tree and shake the branches and I guarantee you'll find it very difficult to breathe with the cascade of dust that is too far away from all but the most violent rain. Years and years of dust will coke you, I absolutely guarantee it. That factor alone would hasten the shedding process of less efficient inner leaves reducing the active foliage. Measure that!
Bugs love dry hotels, especially the evil little bastards, mites. They are an ill wind that does no one any good.
All-in-all, a lousy trade off likely not worth the discussion.