All other factors aside (legality, who gets what etc), what should really matter is how you feel at the end of a multi sector 15hour duty.
At my last airline, we regularly did up to 16 hours, sometimes with minimum crew and no rest period - and occasionally this was extended. Our rest period after that duty would be as little as 12 hours sometimes, and it was not uncommon to do back to back days of 14hours, 14 hours, and 12 hours, all 4 sector days.
I certainly know that I felt like absolute crap after just one of these, let alone being able to look forward to the next two days.
(on a side note the destinations and pay were no where near as nice as QF LH).
The important issue is fatigue, and I thought that airlines were going to have to introduce a fatigue managment system?
The point is, how would you feel evacuating a full flight in the rain, in the dark, after 15hours at work. Or how would you feel fighting a fire, or dealing with a medical emergency, or responding to a depressurisation after 15hours at work?
From my experience, I can honestly say it wasnt nice.
Now CASA may say it is legal, but its only legal if the crew are not fatigued. This is where the company i worked for made it difficult - there were very few people that had the guts to say 'I am too fatigued, I am not fit for duty'. They put the fear of rotten rosters, job loss the next time things slow down, and so on. I only ever saw one FA do it - with minimum crew he had been at work 15hours and had another 3 to go after a weather diversion and a tech delay.
CASA also get it wrong sometimes! And they will never know until its pointed out to them, be it through crew who approach them, or through an accident/incident as in my case.