It's a tactical decision obviously, based on the # of lefty's in the Australian side that is. Which is just lovely to see, never really been a fan of our "let's put out our best side and focus on our game" garbage.There is Amit Mishra as well..
BTW Is Umesh Yadav still injured? because I don't see any other reason for him not playing Tests. Zaheer should definitely come in place of Ishant, at least he can't bowl any worse than Ishant. Also what about Ojha? Why he isn't playing too?
As for Australia(this also goes for India and every other country that will go on to lose away tours ), the objective here is to learn and more importantly learn from your mistakes(whatever they may be). I don't know when Australia tours India next, but I would really like to see the Nathan Lyon's and the Phil Hughes' to return, not to be smacked around by the MS Dhoni's or bullied by the Ashwin's of this world, but to have learned from their experience in India and more so around the world.
If you don't plan to bring them back, there's no point of them being here in the first place.