Dre was incorporating rock elements into rap before anyone outside of Seattle had ever heard of Nirvana. If anything he was a fan of their music, but I don't see much in the way of musical influence.
I said nothing about Kurt Cobain being responsible for Dre incorporating rock elements into his music, being the reason hip hop got "dark" or the reason people wear ripped jeans, jesus christ...I never even said he had an overt, direct influence on how people spit raps or program drums, but it seems like some nikkas in here a simple minded.
Dr. Dre, as one of the most influential figures in the culture as a whole being a fan of Nirvana gives you an idea where he sits in the consciousness of Hip Hop as a collective as your "favorite rappers favorite rocker" to flip that phrase.
There hasn't been a more iconic figure in rock music across the time line of the culture for rappers and fans to identify with. If there is a resurgence of rock-rap going on, I would say it's reasonable to surmise that he a substantial amount of influence on it. Two of the more influential artists that you can credit for that are huge fans of Nirvana and Kurt as an individual, I don't see how that can be reduced to zero influence. There has to be a little middle ground here, but this is the coli.