Legalities aside, crossing Bass Strait, all a lifejacket is going to do is give you a false sense of security. You ain't going to last long, as in, all it will do is prolong the suffering.
I believe the Bass Straight survival time in the water is 20 minutes. In my mind altitude a) gives you the longest time between mayday and ditching so that the time to rescue is shorter and b) increases your chance of ditching near a ship or something.
But, you've got to love the question to a 747 pilot about why he flies a 4 engine 747 and the answer being because there are no aeroplanes with 5 engines.