I have a 2nd gen Macbook Pro Retina 15 inch. For pretty much the whole time I have been running it at the scaled 1680 x 1050 resolution (1 up from the "best for retina" resolution).
After randomly playing around with the different resolution, I came to notice that the "best for retina" display is in fact better quality to my eye (so much so that its almost painful to use any of the scaled resolutions now):
I tested this by browsing and reading text at the two different resolutions, making sure to zoom to accommodate for size difference. I even tested it out reading smaller text on the higher scaled resolution with the same outcome. The "best for retina" (1440 x 900) appears less fuzzy and significantly reduces eye strain.
My question is (aside from wondering if anybody else has noticed this): is there a technical reason that could account for what I am seeing?