Honest opinion
I strongly feel that window shades should be closed on flights longer than a couple of hours. If you need lights use the overhead or the "snake light" attached to the seat.
Different people have different needs. Someone would want to adjust to the time-zone of the destination. Another person may not have slept the day before because of work/travel/baby or whatever and was planning to catch up with sleep on the 15 hour flight.
The glare from the sun at 35,000 feet is enough to light up the cabin even if one window is open. This distrubs the ambience of the entire cabin. If you really need light use the artificial light sources inside the plane which have much more focused beams.