I work in aviation and am flying almost every day due to it - I'm a flight attendant. While the simple, off-the-cuff response in regards to electronic devices is the clichéd "interference" excuse, the main reason (and this is coming straight from the training manual) is actually for safety during critical stages of the flight - take off and landing. While it is plausible that transmitting devices may cause some kind of interference, it is preferred that devices are switched off so that passengers can be briefed on safety instructions and so that they are observant. It's the exact same reason why we ask for window shutters to be open too - because the flight attendants obviously cannot see outside the aircraft during takeoff and landing, but the passengers will, and will alert crew if they see anything abnormal (for example, smoke coming from an engine). You have to remember, the attendants cannot see out of the windows as they don't have the luxury of window seats or being able to sit while they work, and the pilots have a very limited view (and their gauges don't necessarily tell them immediately if there's a problem). If passengers were allowed to click away on their devices with not a care in the world, who else is going to observe what's going on outside? Oh wait, no one.
