The reason the lights get turned down are, as has been mentioned, for your night vison should you need to leave the aircraft. It is also done so that the emergency lights in the cabin are visable and pax can see. during an evacuation there may not be any lights or visability in the cabin.
Regarding the window shades, it is only at the most critical stages of the flight when you need to keep them up. (take off and landing as the engines are nearly at their limits).
Communication is very important onboard so the more passengers able to look out a window and report strange goings on, the better for all those onboard.
I think the British Midland flight which crashed years ago on its approach is the best example of bad communication onboard.
Pax and crew are the eyes and ears of the flight deck crew. so keep those heads out those windows!!! many thanks
