Why would you need an instructor rating or multi crew time to supervise a qualified pilot acting in command of a single pilot operation?
... because there is a potential command conflict. The pilot supervising is the pilot in command for the flight, but the pilot logging ICUS is fulfilling the duties. At what point does the pilot supervising take-over, give corrections, how well are they training in monitoring approaches (as opposed to flying them) etc etc
I have read the CASA communication, and see that they "expect" that such training has taken place, but do not require it.
Having thought about it a bit, I don't see this as such a bad thing,
if it is done correctly. In a single pilot context, it doesn't usurp a paid job. It does give an opportunity to gain experience in a commercial operation, which is perhaps more useful than hiring an aircraft to fly around by yourself.