DMX is a hip hop legend.
Everyone knows he's in a questionable place, but X has done so much in the scheme of hip hop history that you don't come out of your mouth with telling him he's cold.
Dude's a fukking crackhead selling 5,000 copies in his first week.... that's not cold, that's done. You don't even need to say. They don't have him on the show, I'd imagine, under the guise that he's a cutting edge mainstream artist any more. He's on because he's entertaining, a big name and he's a legend.
Compare to the likes of, for example... TI. Dude is not legendary status and he's still trying to compete as a mainstream power house. That pretension does not exist with X to where it would be redundant to question whether he's cold.
As far as Stephen Jackson... I don't think he's wack. Just average and generic. He has good delivery and if his bars weren't so meaningless, he would be aight.