Can't quite agree here. My previous aircraft had only a ModeC transponder, and it was used to great affect many times. I used Flight Following a lot, and it was by others who could see me. Was contacted many times by Melbourne Centre to want of converging aircraft - so that's a good thing - AND - Was used to enter controlled airspace (which we can actually do with the right aircraft and gear) Yes. Of course you will need a part 61 licence, which I have. But the fact remains, an RAA registered machine can enter CTA.
ANY transponder is better than none - but I do agree, ADSB is far superior - which my new bird has both in and out.