Currency is slightly more fungible than this. Not enough to invalidate your point, but worth noting that I've purchased food, transportation, lodging, and knickknacks in Saudi/Bahrain/UAE and Oman using USD. All of the above items, in each country. It's certainly not a universal currency though, I'd say the vast majority of vendors were not interested in taking USD. Even then though, generally you could find a private citizen/local expats from many other continents that would be willing to perform currency exchange to/from popular currencies at a rate profitable for them.

Vendors/citizens of some other countries - Jamaica, many parts of Mexico, many parts of Canada, also are happy to deal in USD as a customary business practice, at least in my experience.