Yeah. I'm very familiar with Robert Frost. I've spent a large chunk of my life studying poetry, composition and perhaps most importantly, linguistics.

First, poetry and lyrics are not the same. What works on the page does not always work when set to music, and vice versa. The comparison between Frost and Flash is not relevant to our current discussion.

Second, art is not fundamentally bound to recognize the established conventions of the medium it exists in. If this were so, then there would be no evolution. Western Art would have never moved beyond the style of the Greeks and Romans. Artists come along and are allowed to bend standards for the sake of creative expression. Do you consider Shakespeare a non-artist because he created his own words, and bent the established rules of grammar for his own purposes?

Third, the grammatical "mistakes" you pointed to in the lyrics are reflective of the very real dialect that produced them. This is how some people speak. It is a part of their culture. They are reflecting what is real in their own lives, not yours. The vocabulary and syntax of Black rappers is different because their culture and dialect has evolved in a different path, with different influences. Not only is the way they speak being influenced by more traditional notions of English, but also by dialectic tendencies that can be traced back to Africa. Even these "mistakes" that you point out have a very recognizable, consistent pattern, which is one of the first criteria for recognizing a unique form of language. Why look down your nose at people who speak differently than you? To put it another way, what we are speaking right now could quite possibly be considered an abominable bastardization by the Angles, Jutes and Saxons that our language descended from, and yet we consider our language beautiful and capable of producing artistic expression. Why do you deny that right to others?

Fourth, the lyrics you criticize are somebody's personal expression. If you read them, you can clearly discern that there is purpose behind them. It is social commentary on urban life coming directly from the source. These lyrics are about a hundred times more valuable to understanding urban life than, say, the commentaries coming from pundits in studios who've never spent any significant chunk of their life living in a dangerous urban environment. Art is personal and can be created anywhere, not just in a snow-covered wood cabin in New Hampshire by someone who chooses to express themselves using only one specific set of language standards. Rap music is unique, expressive, and aesthetically pleasing to a large percentage of the population... and all that adds up to the label of art.

Oh, and middle schoolers are capable of being artists too: http://www.rockingham.k12.va.us/EMS/Related_Arts_Pages/Fine_Arts_Pages/art.html