Bus stops are far more interesting and useful places to have art than in museums. Graffiti has more chance of meaning something or changing stuff than anything indoors. Graffiti has been used to start revolutions, stop wars, and generally is the voice of people who aren't listened to. Graffiti is one of those few tools you have if you have almost nothing. And even if you don't come up with a picture to cure world poverty you can make somebody smile while they're having a piss.