I think they were placed into the category because of their immense potential. Culpepper and McNabb were considered the culmination of what was thought to be the ultimate quarterback type: strong, fast, tall and cannon armed. These guys had so much potential that it was automatically assumed that they would be great. I think it took a while for them to mature but both of them, and McNabb especially, have matured into the quarterbacks they were supposed to be.
As for Rush Limbaugh, what he said was stupid. I am a conservative and listen to the man from time to time. This was not one of his finest moments. There may be some in the press that still see bias against black quarterbacks in the league (I don't see how, I think at least 1/3 are black now) but for the most part there is no 'Willie Mays' of African American football players. I personally find that McNabb is a very good role model for most kids. From what I have read about the guy he is very nice, very modest, has a hell of a work ethic and is a team player in every way.