All US cards appear to charge a hidden spread of about 1% on top of any published foreign transaction fees.Thanks. Takes a good chunk out of any earn with visa/mc usually <0.5%, plus fees to pay the statement in USD.
When combined with the spread/fees you lose on transferring money from AUD to USD, you need to be getting considerable benefits out of the card to warrant putting spend on a US card.
For me, it is worth it because I get, on average, about a 15% return on spend (after all these foreign exchange losses), but I only churn. I would never put non-churn spend on a US card. And that 15% figure is calculated conservatively using the cash out rate of US points. If I assign their true redemption value, the figure is closer to 25-40% return on spend.
And of course, there are some cards that can be worth having even if you don't put spend on them (eg Aspire).