Even without billboards, we are constantly bombarded with advertisements. Any time you see any type of product, a car, a shirt, a watch, a bottle of water, a fence, sunglasses, etc.. you're seeing an advertisement. So its not like people would stop buying things if there weren't big ass billboards telling them to. They'd still be exposed to the product, and if its something people actually need they're going to buy it no matter what. Graff can somewhat be an outlash against the bombardment of advertisement. If they get such a loud voice, why don't we? Well we can, bomb your message.
Anyways, I think its a great idea. And yeah graff will stand out so much more, the businesses might start doing guerilla marketing, though, like sony did with the psp.