rm is right. The main reason is so that your eyes get adjusted to the natural light so if the emergency lights do come on, they will stand out a lot more.
My current arline only requires the lights to be dimmed during the hours of darkness, but at my last compay, it was a requirement for ALL take offs and landings, which I think is better. But that is my only personal opinion!
I can see your point about the reading lights though, although the cabin is still much darker than normal.