Not to create more discussion on this since it is not really the point in case... However lets assume there was some kind of incident, no smoke without fire, bad pun I know but it validates my point. The very last thing the flight deck need, even if no worse is 'dit...ditty...dit...dit....dit...ditty...dit...dit' cutting across thier (possibly emergency (no drama intended)) RT.
Quite right and reasonable to point out the flight mode feature, however its is only valid if the device were already set to this mode BEFORE departure otherwise it would have to be switched on and would then try to 'touch base' with a cell before the new mode could be selected.
The rules are still the rules and in almost all cases say NO MOBILE PHONES, 'flight mode' or not!