Trump
Newsweek has a piece on Trump and the Apprentice that helps to explain why certain contestants seem to stay around despite giving every appearance that they do not work and play well with others (this means you Omarosa):

The fact that wacky Sam lasted on the show as long as he did has made people wonder whether Trump, who is an executive producer of the show, keeps the most entertaining people even if they're the least competent. He has also spared the show's reigning villain, Omarosa, even though his own lieutenants have recommended she go. "It has nothing to do with the fact that we have great ratings and I want to keep them that way. It's just pure instinct as to who's going to do the best job," he says. "Wait until you see me go after Omarosa." Not everyone is convinced that Trump is playing fair. "It's a TV show. You gotta remember that," says fired contestant Bowie Hogg. "Trump knows how to get good ratings. He's a smart, smart man."

I guess its just good television to keep the villians around. (Found via Businesspundit)