I was also amazed to read that folks that DO have transponders turn them off! I realise the need to have a circuit breaker, but what are the penalties of making the c/b unavailable to the pilot? This is a serious question - don't just flame - tell me why the transponder cannot be on all the time - just like the sidelights on a Volvo car?
Transponders can and do malfunction, on all types of aircraft. A misleading reading needs to be switched off. Apart from that, I can't think of a good reason for not having a transponder on.