How common is the knowledge that you can build extended stopover transits into itineraries?

As you say, it doesn't always work. But last December we went AKL-SIN. Then flew Air Asia (no comment) from KUL to Indonesia as it afforded us a trip through Peninsula Malaysia and no need to fully backtrack to SIN.

With the knowledge you've imparted upon me through today's discussions, Air NZ's multi-city engine suggests that I could have instead booked AKL-SIN-long stopover-CGK and essentially got a "free" flight SIN-CGK. Full service airline no less. This seems totally nuts.

The only reason I'm not completely kicking myself is that the opportunity cost would have been having to backtrack to SIN from the top of Malaysia where we ended up.