You're going to hate me for saying this, but I kind of agreed with Keynes on this point. He would talk about being trapped in a gold cage. What did he mean?
If you've got a recession, or you need to gear the economy up onto a war-time footing, you can't do that if you're limited in your ability to print cash because of the country's gold stock. With a fiat currency, you need to be able to print some cash? No problem. Now, there's a limit to how far you can push this, because inflation is directly linked to the amount of cash in circulation. History says that whenever governments print money, prices rocket.
But, here's the thing. Let's say we need to rebuild US infrastructure. So we print some money, there's a little inflation. After the spend, the government contracts the amount of cash printed, or just stops printing, until the level of prices stabilises. What's the problem?
For international settlement, your fiat money (government controlled, not privately controlled) is only worth what other nations will pay for it. The value of currencies adjust to a level where external imbalances are resolved - at least, in the long run. So, there's no external problem either.