Isn't there a risk if a mobile in use is dropped, that the phone-battery connectors could spark?
People juggling hand baggage, phone or petrol bowsers are more likely to drop the phone.
The batteries on some mobiles become detached when dropped. If detachment occurs while the phone is on (even on standby) a spark could occur. Same is true with any hand-held battery operated device.
Best reason for phones being off when boarding is that some people have obnoxious loud conversations disturbing to other passengers and make them unable to hear any announcements.
Kids get run over by trams when crossing roads due to phone/Walkman in ears. People need to focus properly on what they are doing, when they are doing it, to reduce risk.
Just like the axiom of Aviate, Navigate, Communicate.