The reason that radios and avionics are turned off is to protect them from the high voltage back emf that appears when first connecting a DC motor (the starter) Fleming's right hand rule I believe. Most modern avionics have internal protection but some older equipment does not. Bulbs are not susceptible to such transients. The same reason that many car starter switches disconnect the radio when operating the starter.
Last edited by Whopity; 21st Sep 2011 at 08:11.