As I said, it's really only useful if (a) you're travelling to the USA, or (b) you really, really need the money.
If it is just going to sit there, it really isn't worth a lot.
Since the greenbacks were versatile, even when the AUD was doing better than it, the folks bought lots of it at the time. But it's not an immediately useful instrument (although much more immediate than shares or gold bullion, admittedly).
I'm in a funny similar dilemma owning CHF. Fairly versatile currency right now - do I retain the CHF and maintain its relative versatility (even if it weakens against other crossrates, it's bound to still be better than at least before the unpegging) or do I convert lots of it to AUD and make a "windfall" but otherwise end up with a currency that is relatively weak and I don't necessarily have an immediate use for?