An ad that pretends to be art is - at absolute best - like somebody who smiles warmly at you only because he wants something from you. This is dishonest, but what's sinister is the cumulative effect that such dishonesty has on us: since it offers a perfect facsimile or simulacrum of goodwill without goodwill's real spirit, it messes with our heads and eventually starts upping our defenses even in cases of genuine smiles and real art and true goodwill. It makes us feel confused and lonely and impotent and angry and scared. It causes despair.