No requirement for an I/R to hold CPL in Australia, unlike most other countries. That's why you have many students doing CPL first, and adding an I/R when they can afford/actually require one.
Nor is it required in the USA, I don't think it's required anywhere actually.
But what use is a CPL without an IR?
Additionally, you have to build hours to get enough for the CPL, why not build them doing IR work? two birds, one stone kind of thing.
Seems silly to get the CPL first, then spend more money getting the IR, when you could have got the IR on the way to the CPL and be a better pilot for it.
But still, not the real point of this discussion.
MOST congested airspaces require specific procedures for traversing VFR. If you want to travel through congested airspace, take the time to learn what is expected, then request that, and most of the time you'll be accommodated.