I flew with someone recently who was a relatvely low houred pilot. When we were about to take off I asked him about the transponder being off. His reply was that we didn't need it as we were only going to be flying locally.
To me that is a baffling attitude considering the plane had a fully functional mode C transponder fitted, and I can only imagine it was down to his PPL training.
I fly with Mode C and the new aeroplane has Mode S and also use traffic avoidance systems regularly, and really I think it is stupid to have an aeroplane with a txpdr and not turn the thing on.