I'm not a huge graffiti fan, but quite a few of my friends are. My personal view on it is similar to Helm's: That it should be used for objection to political, enviromental, and cultural issues, rather than: 'OMG, a blank wall! Let's spray it with a three letter "tag" rather than actually put some time and effort into it!'
But yeah, I'm fine with motifs etc., just not the things that actually ruin the scenery. I was close to tears (well, sort of) when I found the same 'tag' all over the park. Trees, benches, nothing was spared. Then again, I take it that you live in the USA, so the grafitti culture over there might be a bit different to how it is over here.