Graffiti is the antithesis of homogeneity.
I came back to this thought recently because of noticing the variety in quality of graffiti and its effect on the urban landscape AND vice versa. Graffiti is not necessarily a negative or positive value but a multiplier.
It reflects and magnifies the urban landscape and its qualities, good or bad, for better or worse. And even in the worst cases, graffiti simply calls attention to where urban design has been neglected or abused.