Transponders are generally turned on and off every flight. ATC don't need to see a bunch of aeroplanes sitting on the ground at the terminal, so they are turned off after engine shut down and turned on just after start up. Using the standby switch is not a once in a career event, it happens all of the time. On the other hand, turning the transponder off in order to be undetected by radar is a very rare event. We should design systems to be useful everyday, not design the usefulness out of them just to avoid a one in a billion event.