I write because your website is informative, honest and useful and therefore one of the best places to seek explanations for the US basing more 2500 troops in Australia. Your current explanation however is somewhat lacking, for reasons that I will detail below. To help you I have complied a list of things that are NOT the reason for these troops being there.
First they are not there because Australia is a weak country desperately needing defence against enemies with the ability to invade it. Whilst Australia isn't the greatest military power in the world it's army is not only more competent man-for-man than any in the region, but more competent than the US army and "battle hardened" thanks to the Iraq and Afghanistan stupidities. It is also big enough to handle any realistically transportable invasion force in the region. This is hardly relevant however since any invaders would have to overcome the Royal Australian Air Force and Royal Australian Navy before the first digger even puts his boots on. I do not say that the Malaysians, Indonesians, or Singaporese cannot come, I only say they cannot come by sea. Which is where they'd have to get their supply from, given the place is a desert. So they'd have to get and keep air superiority against the best air force in the region for at least months. This would be even harder once we get our fancy new massively over-priced planes. If the F-35 isn't sufficient to defend destroy vulnerable troop transports we should ask for our money back. Even if a couple of thousand troops were necessary to forestall invasion Australia, (unlike some countries) has total government debt about 1/5 of it's GDP by IMF accounting and can thus easily afford to hire more.
The second possible reason is that they are there to counter China in it's attempts to control the South China Sea, a vital trade route. There are several reasons why this is not the case not, not the least of which is that armies don't float. You can't control a sealane with troops, unless it's the Strait of Hormuz and they have missles. Australia's over-budget, underperforming submarine fleet is far more capable of influencing events on the SCS than these troops will ever be. A far bigger problem is that why the hell would we care? By "we" I mean Australia. The main Chinese strategic interest in the South China Sea is to keep it open to trade, much of said trade being us digging up half of Western Australia and the Northern Territory then selling it to China. The only way US troops could help Australia's interests in this context is by refeuling Chinese ships when the Aussie troops take a sickie.
Another strategic interest is the control of energy resources around the Spratley Islands which are contested. However considering the estimated peak oil in the Spratley Islans about on par with Vietnam and the Natural gas fields are about rich as Thailand's. I hardly see this as worth getting into a shooting war over. If it was worth getting into a shooting war over it would be between China, Vietnam, the Phillipines, Malaysian, Taiwan, and/or Brunei. Neither America nor Australia have a dog in this fight. Even if it was worth committing US troops to such a conflict it would make far more sense to station them in Taiwan or the Phillipines. Not that you'd even need troops to win such a fight since it would be an air/sea battle. Given that neither country appears to want US troops why should the US bother to defend their claims for them?
A third non-reason is that they're required to allow the US to invade nearby countries. While 2500 troops would probably be sufficient to occupy Papuan New Guinea who the hell would want to? The forces are nowhere near sufficient to take and keep Malaysia or Indonesia, even assuming neither went to the aid of the other. They might be able to conquer Singapore but selling Singapore as a threat to world peace would be beyond the capabilities of the most deluded neocon. None of this matters in any case because any invasion from Australia would have to get at least tacit Australian approval, and a shooting war along one of our biggest trade routes is to say the least, not in our interest.
Of course it's possible that Obama wanted the troops to have somewhere nice to stay. The cournterargument is they'll be staying in Darwin.
If anyone can come up with a semi-rational reason why it's in the strategic interest of America, Australia, or indeed any country to post these troo