As a former Part135 pilot I can tell you the reasoning for the rule....
In the OLDEN DAYS - planes lacked significant EM shielding and one of those 3 watt cell phones could play havoc with navigational gear and actually destroy an old LORAN C
I have seen a pilot talking on his cell at 10k feet on a descent telling his wife to head twoards the airport.... ( wait did I just say that )