I think the basic drive behind graffiti is literally to "make your mark." Most graffiti is made by people who feel ignored, left out, with no access to other channels for "publishing."
I usually enjoy a funky, graffiti-splashed wall better than the conventional urban techno-clean look, myself. And I dread living in a city where anti-graffiti controls are strictly enforced, a city which will also probably ban posters, loud music, street performers and any group of people standing around on corners.
But I think that we have to address not the symptom of graffiti, but the root cause, that some people feel so alienated from their community that they must deface its walls. In some cities, certain walls are designated public, graffiti-permitted spaces; or youth groups are paid to create public murals, giving them a sense of ownership of the space so that they patrol it themselves.
The "broken window" window theory, that where one mark goes up more will follow while clean spaces are more likely to remain clean, is not regarded as universal truth. Some taggers regard newly painted space as a challenge; the more aggressive anti-graffiti forces get, the more aggressive the taggers get. Even murals created by street groups cooperating with the city have been tagged and defaced by other groups that don't feel like cooperating.
There are business owners who will never be comfortable with "funky stuff" in "their" neighborhood, and certain angry and alienated people who will never feel like cooperating with the rest of a community that they feel rejected them and looks down on them. Social justice cannot satisfy everyone perfectly, but can only try to satisfy everyone equally. An increased sense that we all own our community, have equal rights in it and an equal voice in its appearance, may decrease the sense of entitlement that any one group or individual has to control what everyone else has to look at. Building that sense of community, however, is not easy in our increasingly balkanized cities.
Nothing is easy. That doesn't mean it's not worth doing. Fortunately, I see a rise of community art and writing programs getting youth and other folks on the street involved in alternative ways of self-expression. Check out StreetLife Gallery.
Write On! -- preferably on paper. :)