It seems to me that when people make statements like "art is pointless" in a setting where it is obvious that art is your passion and livelihood, it is done with the intent to put you down, and to try make themselves feel somehow more important. They are often very insecure, and use rudeness as a way to empower themselves. I don't have much tolerance for that behavior. I would smile sweetly- acting oblivious that they have just tried to insult me- and tell them that "Oh, well the door out is the one you just came through". I find that people that behave badly like that don't expect a return confrontation and they often apologize and slink away or they totally turn around become ( outwardly at least) quite pleasant. If they were to persist, I would tell them that art makes me happy and makes me smile, and that I hope that something else in life does the same for them , and then walk away to let them think about it !