As an engineer, the scenario of interference from handheld portable devices causing problems for modern airplanes is not realistic. However, the real danger that is being mitigitate by turning off all portable electronic devices during takeoff and landing is that they don't want there to be ANY distractions should anything go wrong. In the event of an accident they want to have the full and undivided attention of every passenger in order to communicate what to do. Every second that you save while trying to evacuate a plane is critical. That's why there is supposed to be nothing blocking your feet while seated.