Christo and Jeanne-Claude’s Gates stir all kinds of associations, but perhaps the most important thing is that they became an occasion for the city of New York to unite. They spawned a grand, gala, inspirational event: Happy crowds came to witness the first unfurling, Mayor Bloomberg seemed uncommonly emotional, and they generated a good deal of business. It may seem cynical to mention the last—much touted as proof of Bloomberg’s business acumen—but it points to the fact that New York, which has been under economic siege since 9/11, once again was in the black. Thus art’s catalytic and rallying power,...