Exactly. In addition, keeping passengers calm and relaxed is also a key point of security - roudy/drunk/agressive or even violent passengers can cause a lot of

Exactly. In addition, keeping passengers calm and relaxed is also a key point of security - roudy/drunk/agressive or even violent passengers can cause a lot of trouble on a flight when there are relatively few, fairly untrained flight attendants on board. Thus, if everybody was chatting away on their phones with annoying ring-tones going off every 2 minutes this would probably start aggravating people, so its just easier to say 'no mobile phones' - and if people have the impression it is a point of safety then they are more likely to obey them.