While we're on the subject of ratings, here's an anecdote I've always carried around with me. Tehe PG-13 rating was the result of "Indiana Jones and the Temple of Doom", mainly due to the heart scene. Probably too intense for 8 yr olds so they had to provide a more guidance to parents.

NC-17 came about because of "Henry and June", mainly because of an erotic lesbian scene. I would say that 95% of the movie was typical R drama, but everyone would be focusing on the one or two erotic scenes that it would distract from the whole (it really was a fairly boring, uninteresting movie). So a NC-17 alleviated that problem by just targetting it to the art/indy crowd, where quite a few NC-17 movies can do well.