An arming switch/ device is not bulletproof either. It is just one layer of safety.
A plane can still get away with an arm/disarm device. Having multiple layers of safety
and using them all the time is a sure way to avoid an accident. Like I posted above I do not use an arm/disarm
device simply beause I have multiple layers of safety and use them religiously ( and I do not like the way arm/disarm plugs look outside a plane) I did
loose a 2 meter ship in a runaway situation ( and I am not the only one believe me ). It was my fault-in my scenario an arm disarm
device would have not prevented the loss from happening . Here is what happened-immediately after landing I grabbed the airplane by the vertical fin to bring it to the pits as at our
field taxing from the runway thru the flightline to the pits is not permitted. As I placed my plane in the pits before disconnecting anything I pointed the nose away towards the runway ( I
always do that). I had installed a Digi-switch inside the plane ( it looks quite big and ugly outside) and before removing the hatch to turn it off for some unknown reason I switched my
transmitter off before disconnecting the battery from the ESC. Plane went to full power immediately thru the flightline towards the runway, and on its way it swerved and hit a startup stand,destroying itself in the proccess. Took less that 2 seconds. All blame is on me . I had not developed and tested a system to prevent the situation from happening. Like we say in aviation - all the holes in the cheese lined up - thus allowing the incident to happen. So just relying in the arm/disarm device is like an open invitation for disaster. Here is what I do -
1- Transmitter set up with a throttle inhibit/cut switch
2- Failsafe at transmitter set and TESTED !
3- Always point nose towards a clear path away from the pits,spectators and objects.
4- Always keep thumb holding the throttle at idle even with the inhibit switch is active.

An arm/disarm device would add extra safety. I just dont like them. When seting failsafes they should be tested for proper operation. That is what I failed to do on my incident. At D3 pattern contest we have made everyone arm their planes , hold on to it,throttle up some and then turn off the transmitter . A few motors went to high power, others kept spinning and some went to cutoff. All could be blamed to improperly setting up the failsafes and FAILING to test it ! This is a very important step that should not be overlooked.
So do not just rely on an arm/disarm device wether it be a plug or switch but have a system with multiple safety layers and test them ( failsafes) to make sure it works...