The thing to consider about Sabrina and Mel is that while they have dance experience, it is "pop" dancing, choreography, and very different from ballroom. The foot position, body posture, and types of moves are all very different. Even the muscles used are different. And in order to do ballroom, they have to "un-learn" everything their bodies are used to doing for their show/music work. The only real advantage I think they have is that they are used to live performing, and know how to deal with stage fright. Watch "so you think you can dance" and see what a hard time the classically trained dancers have with the hip-hop moves and vice versa to see what I am talking about.
And I think Emmitt Smith must have had a lot of disco dancing experience. Those moves came pretty naturally for him. I don't care what their background is, I just enjoy watching good dancing. But to each his (or her) own, I guess.