According to the FAA, there are still unknowns about the radio signals that portable electronic devices (PEDs) and cell phones give off. These signals, especially in large quantities and emitted over a long time, may unintentionally affect aircraft communications, navigation, flight control and electronic equipment. The FAA has issued guidance to airlines letting passengers turn on most PEDs after the plane reaches 10,000 feet. At a lower altitude, any potential interference could be more of a safety hazard as the cockpit crew focuses on critical arrival and departure duties.
Boeing, maker of airplanes states that: operators of commercial airplanes have reported numerous cases of portable electronic devices affecting airplane systems during flight. These devices have been suspected of causing such anomalous events as autopilot disconnects, erratic flight deck indications, airplanes turning off course, and uncommanded turns.
When we asked veteran flight attendant and author Heather Poole why the cabin crew instruct us to turn off our PEDs she added in addition to the reasons we've already listed that they are asked to be turned off, "because one phone may be OK left on, but 160 (the number of passengers on board a 757) will interfere," it is "so that passengers are paying attention in case there is an emergency," also "so that the devices don't get in the way if there is an emergency," and lastly "so that passengers are able to move quickly if there is an emergency."
As you can see there are clearly some very legitimate reasons for the law, besides it's the law. While we know it is important to obey FAA regulations, it is nice to know the reasons behind why we are asked to turn off certain PEDs during takeoff/ landing and are asked that cell phones be turned to airplane mode during flight. So hold your Words With Friends and Angry Birds action when being told to do so by cabin crew because while you might not think it's worthwhile, be a good human, obey the law and respect the others on the plane.
For additional reading, check out this fact sheet from the FAA.