I wonder whether the poor unfortunates who have lost their lives in the San Diego mid-air thought that having a transponder or even ATC service was keeping them safe.
Just read about that horrible accident. There is little doubt that had they had a traffic info system - which would have worked there - it is less likely that this accident would have happened. Call me a chicken but now I tend only to rent aeroplanes with these devices when flying in SOCAL (seeing another aeroplane quater of a mile away coming towards you, hidden by the wing tends to put the !!!!!s up you).....You just cannot actually see some traffic if it is hidden say by the wing.