There are basically 2 reasons that s/o's are employed by QF:
1. To extend flight time limitations beyond that provided by 2 pilot (domestic, SE asia ) operations and, as a consequence
2. To increase the company's productivity e.g. LAX - MEL 4 pilot (well 2 + 2 !!) is a darned efficient operation as it stands... 350 pax a quarter of the way around the world for similar costs that other airlines operating NRT - SFO or JFK - LAX with a "heavy" crew. Perhaps consider the alternative of LAX -SYD, then new crew SYD - MEL. This would be a huge increase in crew costs.