I think most people dismiss street art as illegitimate but I think it is as credible as any other art form. One of the artists from Lastplak - a Dutch graffiti collective - put it best:
"If you read a book you form your own image of the book and you hear the voices of the people with your own ear. With art it's the other way around. The image is already there but the concept and the philosophy of what it means is shaping in the viewers' head and he's got his own idea of what the meaning of something is."
Check out their website. They do amazing work. Here's the original article from Juxtapoz (great art and culture mag. check it out).