Pace has a valid point. How does one protect themselves and their passengers when you simply have to rely on the Mk1 eyeball and all it's failings? In a Jet at 250kts at 6000' in receipt of a radar service, but HAVING to be outside CAS, one would feel pretty pissed off if one then slams into a glider because no one knew it was there (ref. Hawker 800XP and Glider outside Reno last year). I say Glider because chances are a Light aeroplane with engine may show a primary return on a radar.
There are several solutions to this: a) Increase CAS to keep people out, b) Mandate that everyone fit transponders so that the likes of Pace in his Citation can have fair warning of someone doing steep turns at 6000 just below the airway he is about to pop out of, or c) Come to a compromise, for example all aeroplanes in IMC MUST transpond? or d) make aeroplanes ighly radar visible without having to have a transponder fitted (i.e. radar reflectors) or e) have Transponder mandatory zones in certain busy areas.
I'd vote for e) myself.