isn't everything we capture with a camera...someone ELSES work in a sense?!?!   Unless YOU actually CREATED the subject matter  you're shooting that is.

Photography is  all about capturing a moment or creating a visual document of a time or place.  Graffiti can be considered modern  Hieroglyphics or Cave art in ways.  Its a way of communicating or expressing ones self....its also used to mark territory.  There's a thread over in the copyright forum debating if pics of graffiti could actually be sold.  Since most of the time, it IS an illegal activity, you probably wouldn't get an artist fighting for infringement over it!  

A camera enables us all to peer through each others eyes,  see the world from another view. Y'know,  I think Simon hit the nail of the head when he said this is a SUBJECTIVE matter. 

I also have to echo Kort's statement and I'll add if something GRABS your attention or envokes something in ya....go for it and take the pic.