Well, that's a great question.. I don't know the answer, but I leave mine on at all times. Per Ken Rockwell's page, he explains it pretty well:
Introduced with the Nikon D3 and D300 in Fall 2007, "Automatic D-Lighting" is Nikon's lingo for how we set the Nikon D7000, D5100,D5000, D90, D300, D700, D3 and D3X to adapt their dynamic ranges automatically to fit whatever highlights and shadows we throw at them.
I call this ADR, for Adaptive Dynamic Range. Nikon calls it "Active D-Lighting," which makes no sense. I'll pass on Nikon's words. When you read "ADR," you'll only find it in your D300 or D3 menu as "Adaptive D-Lighting. You could also call it the cuter "Adaptive Dynamic Range Accommodation" (ADRA), but I won't go there.
ADR visibly improves my biggest complaint against digital cameras. This feature automatically identifies highlight areas that will be lost, and does its best to bring them under control. It works, and it works automatically. You have to activate it in the menus, which I'll explain below.
Unlike the impractical Fuji S5, this function just works and doesn't slow anything down.