I don't understand this logic. If I was uncomfortable using the entire width of my monitor, I would have gotten a smaller monitor instead. I find moving my eyes less annoying than moving my fingers to scroll; I often don't even rest my hand on my mouse or keyboard when reading, because I can use my hands for other things at the same time, or even just rest them somewhere else.

My point isn't that everyone will agree with me, but that all of these experiences are subjective. The only actually objective signal that the UI developer has is the width of the screen, so that seems like what should be used. Some amount of margin isn't unreasonable, but asserting that my display is "too large to be comfortable" is dogma, not science.