Omg it is such a pet peeve of mine when travelling when I see someone refusing to actually exchange money into the local currency and trying to pay everything with USD. Like buddy what makes you think this random Peruvian coffee shop is going to have change for your $20 bill.
Mood! I could understand an emergency "I messed up, would you take US$100 for this $20 taxi", but a bit of currency is so useful--especially in countries that aren't tap-to-everything. (Singapore, I could've skipped it. Greece, almost everyone had two prices.) At worst, yay for a cheap souvenir!