Since the majority of my riding takes place in early morning or late afternoon, when the sun is lower, lights on the rear would just blend into the glare. (Unlike most, when I was driving and encountered a high-glare, low-viz situation, I would slow down, being unsure of what was in front of me.) More light added to an already-intense light blitz like that is, to me, useless.
Midday, on fun or utility rides, is different; overhead sunlight creates less glare.
I do see the advantage in light-to-shadow transitions, though.