I assume by "electronic gadgets" you are lumping absolutely everything into it - MP3/portable media players, laptops, games consoles etc...
In which case it isn't always "against the rules, waste of money, waste of time".
Many airlines allow the use of these items in flight (usually, for laptops and games consoles, with the prerequisite that wireless devices (bluetooth, WLAN) are turned off). And I'm sure many people would prefer to watch their favourite DVD than the IFE from time to time... Or learn a foreign language on their MP3 player... Or keep the kids quiet with their favourite game... etc etc etc