What do you think of the Ian Curtis wall? For decades the council fought a battle with the artists behind it, now it's official and part of Wellington culture. I'm sure if Banksy painted a mural the council would be pretty happy. But with a few very rare exceptions (like the two above) it should be done with the council and building owners consent.
Street art and murals give a city it's identity and makes otherwise dull buildings interesting. Writing Pork everywhere is just dumb.