Not that I'd use an MBA for DJ-ing, but last night this bar I worked in was so dark where I was set up, I'd have killed for a backlit keyboard. (Though if our Karaoke software had a Mac version, I'd try it on an air just to ditch the winblows machine)
Some people like to sit outside on their patio at night and it might be dimly lit, or they don't want to sit in a flood light. (Me for instance.)
People that do work in a coffee shop (like 90% of the people in those places at night) where it's ambient lighting is dim
Students taking notes in a lecture hall where the lights are turned down.
*I think that a lot of people forget that not everyone is in an office all day long with their computers. Just because you don't see much use for it, doesn't mean 20 million other people don't almost find it a necessity.