"I don’t define myself by my non-belief in Santa Claus. So why should I define myself by my non-belief in any supernatural being that is currently believed by most people in the world?"

By 1740 BCE the Egyptians had a symbol for zero in accounting texts. Even 'nothing' has to have a name, otherwise you can't discuss it. And yes, considering yourself an "A-Theist" is perfectly legitimate for the same reason we need zero. If asked "What is 4 minus 4", how would you answer? In the same vein, when asked "Which of the multitude gods do you believe in?", you are left with no choice but to say you do not believe in mystery people, and you are an "A-Theist".