Over-bright displays relative to the ambient light are definitely an issue. I've found myself feeling much less strongly about light vs dark since I installed some LED strips on the back of my monitors as bias lighting. (I didn't want to dim my monitors much more because you do lose contrast when the backlight is low)
Contrast ratio is largely independent of backlight brightness. Intuitively this makes sense, because the panel is essentially just an adjustable light filter, which you'd expect to behave linearly.
