Have to say, flying regularly as PAX on the fly-by-wire A320 series, I would prefer a straight rule that all these things be turned off completely. The scope for interference is small but significant. I remember reading of an incident where a video camera - turned on in the cabin with the Captain's permission was followed by with/loss of the nav display on the flight deck. Don't know if cause and effect was ever actually proven, but ...
In my experience, passengers, especially business passengers, have little concept of why transmitting devices are required to be turned off in flight. It's not just about interference with aircraft systems (of which there is a real risk) but about locking out large swathes of the mobile telephone network on the ground. Even in my PA28, it is illegal to have a mobile turned on. Such passengers view the requirement to use flight safe mode as an interference to their busy work schedules and just cannot resist sending that 'one email'. The cabin crew cannot check all these devices. If they were required to be off - completely, everyone would be safer.
Rant over,
HH