Samuel L. Jackson plays Stephen’s overblown insouciance and anachronistic mf-bombs to great comedic effect. There are moments, however, when ironies cancel each other out, and we’re left with a stark truth—at its most basic, this is an instance in which a white director holds an obsequious black slave up for ridicule. The use of this character as a comic foil seems essentially disrespectful to the history of slavery.
Which is, of course, why Tarantino did it. Because if he hadn't, he would look back and see that outside influence/judgment had affected how he made this movie. And in his opinion, the movie is better with that character in it (I agree) -- so he put Jackson's character in. To someone like Tarantino, who really couldn't care less what people think, it's that simple.