You'd have to be a special kind of crazy to fly from Australia to S.America via MIA when you have SCL and EZE as options, and even DFW would save you hours.
What MIA does offer apart from its own reasonable O&D traffic is connections to the Southeast U.S. and Europe on both AA and partners.
No Australian in their right mind would go to the trouble of applying for an ETSA & transiting a US airport to travel to Europe or South America when there are easier options. Not to mention that there are significant numbers of Australian Permament Residents who's home countries are not in the ETSA program & would therefore need to apply for a full visa just to transit an airport....
There would be a bit of holiday traffic to Mexico, mostly CUN, which would be best served as an AA connection from DFW, or LAX for the Pacific resorts.