In my view, the deficiency with only speaking up when they report an unidentified aircraft is , if you have two VFR aircraft without flight plans, ATC will never make that announcement. They will only report it if one is IFR or has flight following (perhaps an ATC staffer can correct me if that is wrong)
In terms of chances of mid air, twice flying from Port Macquarie to Scone I have had close encounters, not near misses, but close encounters, as the cloud on the ranges often forces aircraft to fly at variable altitudes. I have only flown that route about 6 times.
In one case ATC advise me as I had flight following , they advised about 10 seconds before we passed, and I had not spotted them. About 100ft in altitude below me and 200ft to my left. In the other case we had no idea, I had made a position report with altitude around 10min earlier on correct area frequency. This one was about50 feet above me and to y right 200-300 feet. Both these I was at 8500 feet heading west on a corrected AREA QNH. In both cases all aircraft had limited options with altitude due to a lot of localised cloud.