While I won’t deny the talent of some graffiti producers nor the beauty of some of their work, if I pay to have a wall built on my property and I select the materials and any paint or decoration that goes on the wall, I would be big-time pissed off if anyone comes along and puts permanent paints on it, no matter how wonderful the work. Note that I have already had to be certain my wall meets any standards required by where I live and my neighbors.
Towns that provide surfaces for graffiti are doing a very good thing. Sure, it might take the “lure of the forbidden” out of the artist’s experience, but it is still free expression, art, attention-getting, and so on, without imposing on some poor schmuck who wants his wall to be one solid color, or who spent a long time picking precisely the brick color and texture.
I must admit that as a bystander I do sometimes enjoy the colors and patterns of graffiti on train cars as a welcome change from watching several hundred identical tankers go by, but then I am not Union Pacific and I don’t own those cars.