Graffiti is classed as 'street art', and it all depends on individual taste to whether you like or dislike it.
I'm all for graffiti, and spend some of my class time sprucing my books up

. I like to see Graffiti on old buildings as it gives them style, and makes them appear more life like. I also like to see it on the out skirts of the city centre, for the same reason.
Graffiti can be argued as the same reason 'street skating' can, it's a crime to some, but in other peoples eyes it's an art.
Something that takes skill and practice in my view is an art, as you can't just pick up your spray can and create a work of art. For the same reason you couldn't pick up a paintbrush and create the 'Mona Lisa', it would be impossible.
In conclision, graffiti can be argued for and against. But I'm all for it, for the simple fact that it takes time, effort and skill. Not to mention it gives an image of what the 'artist' is like.
Regards, Vibes.