Graffiti is art. What is the definition of art? Art is works of anything that can be appreciated for it's beauty or for it's emotional power. Graffiti is like painting on walls. People use graffiti to express their feelings, and when you see graffiti, you can get a sense of what other people in the world are feeling emotionally.
I could debate both ends for this topic, however I'm leaning toward to the negative side. Graffiti is vandalizing. Why not make your creation on a canvas? But instead destroying personal's properties and use it as a canvas? Not only it lowers the value of the property, it's also give people a negative feeling about area with graffiti.