Grffiti, Urban Blight or Modern Art?

On recent train ride up the coast from San Diego to Port Hueneme, to visit my son, I was struck by the amount of graffiti I encountered. Upon reaching the outskirts of Los Angeles, it seemed like every available surface was "tagged" with brightly painted names and phrases, some little more than quickly sprayed initials or gang phrases, while others were intricately designed murals covering the entire sides of buildings and rail cars!
Since the beginning of Man's existence, we have left our mark on our territory. From the cave paintings of our earliest ancestors and pictographs of Australia's Aboriginal people to the monuments of the ancient Egyptian and Mayan Cultures, there has always been "Public Art". Is Graffiti just a modern expression of this public art? Or is it purely vandalism? Obviously, if it's your property being "tagged" without your permission, it is indeed vandalism. But in recent years, local civic leaders have found it increasingly popular to offer these urban artists a place to display their creations, such as San Diego has done with Chicano Park In Barrio Logan. Here former taggers have turned their talent into legitimate art, telling the stories of their community and culture for all who pass by to see. So it seems that what was once considered vandalism and a cause of urban blight, is now becoming a legitimate art form.