Ah - the old "extra pair of eyes" sentiment. I've operated 2 pilot, 3 pilot, 4 pilot, single pilot, 2 pilot 1 eng, and ummm...3 pilot 2 eng.
Extra pilots are to extend allowable duty times. My experience is that in airliners designed for two pilots, extra pilots sitting in the jumpseats are often as much a distraction as an asset. Historically we have no higher incident rate on our two pilot operations than with four.
I presume QF operate with more than minimum cabin crew (ie in my current airline we usually operate the 777 -300 with 14 - 16 crew vs the minimum requirement for door coverage of 10). So it would have to be very exceptional circumstances for an augmenting pilot to assist with door coverage.
Think about it: why be stuck in some crappy outstation waiting for a replacement crewmember when there is someone perfectly capable of covering a door sitting in a cockpit jumpseat?
Bit hysterical guys.