someone posted this link in one of the other threads. It looks to me that with the backlight off, contrast and sharpness are still better on the Paperwhite than the Kindle Touch.
My guess is that if you are in a brightly lit room you could of course turn the light off...but if you wanted it on, Amazon is saying that strong light is better than weak light.
http://www.theverge.com/2012/9/6/329...ctures#3746597