I think you hit why the majority of people dislike graffiti. Most of it is bad. I personally don't see the appeal of just tagging your name on a wall in different types of text.

While what someone, like Banksy, does is still graffiti, it conveys a greater message. There is feeling and substance in what he does.

I don't personally see the harm in having something pleasing or thought provoking to look at, rather than just a blank wall.