"banksy wouldn't even exist without what filtered out of upper manhattan and the southern bronx and was known as 'tagging.'"

Fair, but we've reached the point where we can discern the difference between something that conveys a message, and what is merely a trademark. A tag is territorial and self serving - a trademark. It might have been what gave birth to other forms of street art, but its value is intrinsically less than something that exists to conjure discussion and consideration, even if the public sentiment towards it is negative.