The possibility of one or two phones interfering with anything on the plane is very small, but if it were to be allowed, the prospect of tens or hundreds of phones all with their 2 watts in use on one aircraft really could create havoc. And who hasen't heard that bip-diddy-bip diddy-bip over the com or nav at some time or other by some GSM phone left on, inadvertently or otherwise? It even interferes with my car radio if leave the phone on the dashboard.