I live near Paris, and it's a shame to see this sort of thing on every surface here. It's so easy and effortless to trash the look of a place, and so much effort and pain to get it back to a presentable state. It just seems hopeless trying to stop it. Sure, you can point to examples of graffiti that don't look all that bad, and I imagine some examples can even be considered to improve the look of a space. But taking…
As for it's quality as art, I don't buy that's a purely subjective, arbitrary opinion (meaning, I think it's reasonable to use some judgment). But people still differ greatly: look at their responses to abstract expressionism, for example; some people think it's trash, others pay tens of millions.
There is plenty of ugly in cities: There is a lot of ugly architecture; buildings are much more visually prominent and for aesthetics I would remove the ugly ones much sooner than removing the street art. There is ugly advertising and marketing; there are ugly industrial sites on beautiful waterfronts and in neighborhoods.
Should those be subject to the same judgement as some kids expressing themselves? The people who make the buildings, ads, sites have far more power and resources, including enough to make those beautiful. They seem much more responsible for the results than the kids, who may have nothing else.
