If you are setting up a plane with automotive 12v ignition, is there any point to having a separate master switch or just use the accessory circuit on the automotive ignition switch? You could still have an avionics switch to isolate anything sensitive, but the master switch doesn't seem to serve much purpose in this case as long as the ignition switch is either the OFF-ACC-ON-(START) or the ACC-OFF-ON-START type. Am I missing something?