Jon, I wasn't all over you as the resident KKK member or anything, I know you have too much brains to be a racist. (Racism is the cult of the inane.)
I grant you that rap doesn't get covered much - that probably has to do with the fact that in rap circles you're nothing if you don't write and rhyme your own lyrics. And that a lot of rap music addresses a certain point or situation in time. No one ever covers Dylan's Hurricane - IMHO his best song and one of his most gripping lyrics - either, that song has done its job, you can't take it out of its periodic context, it would be an empty shell today (which is why Dylan never ever performs it anymore either).
Whether rap is music or spoken poems with some background rhythm is a moot discussion, minimalist music has (and always has had) its place, tribal African drumming doesn't know harmony or melody either, yet we would both hopefully agree that it is a form of music too.
Rap is the voice of black urban youth who rightly or wrongly feel that there is really nobody else who speaks for them. That is why it has its niche. It has always been commercialised and watered down - ever since Sugarhill Gang's Rapper's Delight - and has had its influence on dance music and Euro pop, even quite a bit of rock (Red Hot Chilli Peppers, Rage Against the Machine), but I can't attribute rock's decline to the rise of rap. More likely, white youth has more means of voicing frustration, they don't need to latch on to rock as doggedly as black youths do to rap.
And as regards affirmative action: I infinitely prefer someone being hired because he/she is black to someone not being hired because he/she is black. Once affirmative action has as long a history as slavery and subjugation of black people, we can renew the discussion. In another two hundred years or so?