Art is entirely subjective. There is no real, non-biased standard for "art" that can justify the incredibly wide array of literature, music, drawing, performance... it's just not possible. One person's awe-inspiring, boundary-pushing experience is another's meaningless, self-indulgent tedium. I think Terry Pratchett is the world's greatest living writer, but I'm sure die-hard fans of Chuck Palahniuk or Margaret Atwood would disagree (note: three very different authors, and the fans probably have little overlap, and may easily consider works by the other two to be senseless drivel).
With that said, there should never be censorship of anything, ever. The truest path to knowledge is choice and the freedom to make our own decisions about what is appropriate for us and our families. I do think it would be a good idea for "art" of any stripe to be properly described in a manner which allows people to make informed decisions; if I put down my five bucks to see A Brutal and Misanthropic History of Human Taxidermy with Sound Effects I should be expected to know what I'm getting into.
Censorship is a paving stone on the path to dictatorship; gentle but very, very powerful. Remember the Hays Code for movies in the '40s and '50s? It's still around, but hides under the "Moral Authority" of the MPAA. Watch the movie This Film is Not Yet Rated for more information. Ratings and explanations are one thing; forcing filmmakers to alter their works to fit an undefinable standard of morality (and theater chains that won't accept unrated films) is quite another.