it's simple really...
let's use tupac for example... (the easiest person i could think of that realtes to this)

if you really listen to the emotion and the words of his rhymes... that dude was a lot deeper than what people give him credit for... it was poetry with a street edge.... it doesn't matter what you are talking about... poetry involves emotion and whatever it is you're trying to express from the depths of ya soul... doesn't rap do that? even the hood songs... this is shyt that kats have witnessed and lived to tell about... That song by Pac... "Trapped"... that shyt was deep as hell... ghetto as all hell but it was personal... he talked about his father being locked down... his struggles... that's poetry... but it was also rap...

now i wouldn't take some of Shakespeare's shyt and put a hip-hop beat behind it and call it rap... that would be retarded and there is no way you can leave his words as they were written and drop it on a hip-hop beat and make it flow... if it can be done i'd like to see it! besides, it would also take away from what shakespeare was tryna do with his work.... i could go deeper with this but my fingers would cramp up... it's a lot easier when ya just talking to somebody face to face...lol