Australia is a vast land mass with a low population and massive resources. It is also pretty much isoated. Pretty easy for a greedy power, say China? to invade. The benefits will probably overweigh the negatives in a worst case scenario.
First of all let me say that I served 7 years in the US Army, and am VERY pro military.
With that being said, I wish we would concentrate more on our own borders, then on the affairs of other countries (I'm not trying to sound offensive). I think we stick our noses in other peoples business far too often. We're not big brother, and we need to stop acting like we were appointed that way. We also have a bad habit of spreading our military thin because of that.
They obviously have strategic (usually striking or defenseive) reasons for everything they do. 9 out of 10 times we will not know the "real" reason they're doing what they're doing. They'll tell us either what we want to hear or what they want us to believe about it.
I'm an American and consider myself pro-military. I also understand the need to have bases in other parts of the world for faster deployment and protection of allies, but I don't see the need for a base in Australia.
Is New Zealand going to invade or something?
I think even some places where there are already bases troop numbers should be reduced. The presence of the base at all is a good enough deterrent in many cases, you don't need tens of thousands of troops as well.