Let's start by saying that the QF fuel policy is quite safe. I have no issues at all with them presenting me with a flight plan that is based on the use and carriage of the minimum possible fuel. It's then up to me, and not some faceless person in an office to decide what the actual fuel carried with be. Whilst they do provide what could be described as low level pressure to carry the minimum, that's the end of it. And, quite honestly if you can't withstand (ignore) a bit of low level pressure, then you're in the wrong job.
The A380 that started this thing off honestly had absolutely nothing to do with discretionary fuel, or the fuel policy. I know for a fact that the aircraft departed with an additional amount of fuel. The issues that caused the diversion were a combination of the aircraft burning more than planned, the weather in Melbourne not quite being what was forecast, and runway works that precluded the use of the main runway.
Runway works, associated with a need to carry an alternate, can be so restrictive as to totally preclude the operation. Mostly though, they give an extremely narrow window of arrival fuel figures. A slight wind change may be all that's needed to make it unavailable.