I agree that just because an artist is mainstream, doesn't mean he (or she) is no longer a worthwhile artist. On the other hand, sometimes excessive mainstream exposure causes art to suffer. This is a problem. The prime example of this phenomenon is Nas. It's not all his fault. With excessive success, he is brought farther and farther away from that which made him a success in the first place. Also, I agree that some mainstream artists project values that are less than worthy. The prime example of this is Jay-Z. His music often trivializes women and glorifies materialism and drug use. And even though not all of his songs tread on this subject matter, the one's that are consumed by the largest segment of the public do.

Underground hip-hop also has a few fundamental flaws. First, the recording quality is often poor. Take Benefit's EP for example. It was great, considering he spent very little money, but the quality wasn't as good as a mainstream release. Secondly, most underground artists either lack something that will make them appealing to the masses, or have something that would make them unappealing. A good example of that latter is Aesop Rock, who is an amazingly talented writer who does too much for the casual music fan to take in and appreciate.

So, both sides have their ups and downs, and neither can categorically be taken as gospel.

Peace