While art is subjective, and what is and isn't art will be debated until the end of time, graffiti is vandalism. Why? Because defacing private property is a crime. It's really that black and white. Graffiti is menacing and can create an environment of fear. Graffiti also isn't a necessary component of the "urban experience" and the criminals who tag homes, billboards and commercial buildings should be held accountable for their actions.
People who feel the need to use spray paint to create cryptic symbols and write out childish and nonsensical phrases should practice their craft on their bedroom walls.