I did my PPL out of Essendon, and at that time a high level CTA nav over YMML was required even though every flight was out of class C. It was also required in the test, although I think it would be a harsh testing officer who made you redo because you were denied a clearance.
Yes, they wanted more than just talking to ATC - they wanted you to receive vectors and level changes in CTA.
A few years ago I looked up what was actually required for a CTA endorsement and interestingly, the instructor manual explicitly said a flight was not required - you just had to demonstrate knowledge of the procedures, and that could be done on the ground.
However, flying schools probably have to get their syllabus approved by CASA, and if CASA say you need a high level nav through CTA you probably have to put it in. And if the student cocks it up in their test, and it is discovered that you didn't actually complete it because clearance was denied...