My home town in Sweden has started commissioning artists to paint murals like those in the top image on a lot of otherwise boring and ugly buildings, and I couldn't be more proud. Really makes the town beautiful and unique.
One major reason cities do this is because most fellow spray can wielders will respect the existing work, thus resulting in a nicely painted instead of randomly tagged building.
Some guy recently graffitied my mate's warehouse with the word "CUNT". However, he did it using some outlines and shading, so we couldn't even be mad. Way better than somebody writing their signature in black