Back in the dim dark ages (early 80's), when I worked at Melbourne FS Centre, Mangalore, Cowes,and Wonthaggi were all very busy IFR navaid training locations. There were often multiple IFR aircraft at any one location, all doing their various things and managing not to hit each other. ATS was a directed traffic service from me, and then they worked it out between them. ATC did not monitor, and did not care, even if they could see them on radar, because they were OCTA, and that meant what it said on the tin. Strangely enough, no one hit anyone. How many hundreds, if not thousands of IFR training flights would have occurred in the vicinity of any one of those locations prior to and during the past 40 years? And no one hit anyone. Now two pilots who got the same service everyone else has had ie they were told about each other and left to sort it out, yet still managed to hit each other in what are pretty flukey circumstances. Is that the system's fault, or the pilot's? Or just bad luck? Because that just does happen some times.