Amusingly enough, the expression was apparently first used by Soviet leader Joseph Stalin in 1929 in a disparaging way about the US but was later embraced without a hint of irony by American neo-cons and right-wingers.
It also harks back to Alexis de Tocqueville, the first writer to describe the United States as “exceptional” in the 1830s. Essentially it embodies a belief that Americans are in some magical way a ‘special people with a special destiny’.
I don’t know about you, but I find it very helpful and a great time-saver whenever someone asserts a belief in American Exceptionalism. It’s as if they have obligingly stamped ‘IDIOT’ on their forehead in indelible red marker.
This instant idiot-identification makes it so much easier for the rest of us to completely ignore anything else they may say from that point on.