Basically, for safety. Do remember that if there were to be an accident it is likely to occur during take off and landing. Opened window shades allow both passengers and crew to assess the situation outside of the aircraft (as well as first responders to assess the situation inside).
Edited one time. Last edit by Stafford Vaughan on Dec 24, 2010 at 12:13:47 PM (about 2 years ago).