I heard another reason...
I was once told that the reason you are asked to turn off all electrical devices serves a second purpose. The most likely point of a flight for an accident to occur is during take-off and/or landing. During an accident, there is the possibility of fuel being spread around and coming in contact with passengers. Jet fuel is highly flammable. The less electrical devices that are powered up, the less chance of an electrical device sparking a devastating fire during an incident.