While Parker is handling the creative side of the show, someone needs to pull together the other elements of production. That's particularly true on a show like South Park, where episodes are routinely written and produced only a few days before they air. So while Stone occasionally directed episodes in the early years of the show, he's found his niche as the person who coordinates the episodes, making sure they arrive on time and under budget. This is the business side of things, which Parker can't handle because he's too busy writing and directing. Stone has no problem with focusing on his producing duties; he recently told IGN.com that "I am not a good director, I know that. I am not a very good actor either, and I know it, but it is good to know that."
Another important job Stone appears to have is as Parker's minder, his show-business babysitter. Like a lot of talented writers, Parker is self-admittedly anti-social, and prefers to stay home and work rather than deal with conflict. So Stone does it for him: a profile of the team in Rolling Stone magazine explained that Parker "doesn't like confrontation" -- it's Stone's job to fight censorship and contract battles with the network executives.
And he can keep Parker from doing in life what he does in his scripts. As a writer and as a person, Parker likes to say outrageous things to get a rise out of people. In that Rolling Stone piece, Stone mentions that Parker went up to a woman at a party and proclaimed "George Bush is a great man" just to make her angry. Without Stone to act as a go-between, you can imagine how Parker might say something just to get a network executive angry. It's a relationship reminiscent of Larry David and Jerry Seinfeld in the early years of Seinfeld, where the diplomatic Seinfeld smoothed things over for his volatile co-creator David. Every artist needs someone to protect him from himself.
And, finally, Stone has an important but annoying job: dealing with the media. When an episode sparks the controversy that Parker clearly craves -- like this year's episode where the Queen of England commits suicide and Hillary Clinton has a nuclear bomb hidden in her vagina -- Stone gives the interviews explaining that it's all in fun. And when voice actor Isaac Hayes quit the show last year over its attack on Scientology, it was Stone who went to the press and remarked that Hayes "got a sudden case of religious sensitivity when it was his religion featured on the show." Without Stone to defuse the controversy, Parker might not be able to get away with offending everyone all the time.
What Parker and Stone's relationship demonstrates, then, isn't just that the former is a more talented writer(or, at least, more interested in writing). It's that the less creative partner can perform an almost equally essential role. Without Parker, South Park would never get written, but without Stone, the episodes would never get made, and the show might have been cancelled years ago. In making a good television show, there are more important things than creativity.