I suppose thats the advantage of the 2 brand strategy. When the market is up premium travel prevails and they will use QF and chase yeild. When the market is down they will remove QF and use Jetstar and chase bums on seats with low fares to stimulate the market.
As much as we all bitch and moan it seems to work. QF can launch two brands onto one route and obliterate the competition and stop another carrier like Tiger competing on the route and eroding their own market. QF dictate what the low cost market price is along with the premium.
If you can't beat 'em join 'em.