In general, ratings agencies hurt more than they help. Ratings agencies create an artificial form of censorship, and censorship in society can breed ignorance and squash creativity. However, there is some genuine content that is inappropriate for young people of various age groups, and the rating boards – be they MPAA or ESRB – are the only broadly acknowledged boards that perform this function (whether we like it or not).
Movie ratings do not make sense in many circumstances. However, the G and PG ratings are generally reliable guidelines for parents to determine whether their children should view a particular movie. I know people who use the MPAA ratings for their young children, and there have been only minor complaints.
This Film Is Not Yet Rated (TFINYR) director Kirby Dick argues that films containing graphic sexual content are punished – as measured by their rating – more than films portraying graphic violence, even when that intimacy is shown in an artistic, tasteful, or natural manner. There has been evidence documenting that viewing sexual acts (at least pornographic) does have an impact on the viewer's behavior (link NSFW). However, there is also evidence linking violence portrayed both onscreen and in-game to real-life behavior. Dick is correct, violence should be penalized more harshly in film than it is currently.
So why is it not it given a harsher rating? We are by nature a violent creature. Contest, conflict, and competition are bred into our psyche from infancy with sports and play. As such, people are more accustomed and accepting of violence. In the film industry this translates to more lenient ratings. In the U.S., sex and sexuality are less accepted by the mainstream public as something that can be discussed at the dinner table (unless you're Alfred Kinsey). I agree, sex should be something that people are more comfortable with, particularly since proper education about its benefits and consequences is so important in the age of AIDS, syphilis, gonorrhea, and (unfortunately) many other diseases, but giving more lenient ratings to films that may portray casual or deviant sex is not a way to accomplish that education.
Why don't the filmmakers create their own transparent board of film reviewers and have them rate their movies? I understand that the movies are restricted from distribution unless they have an MPAA rating, but all it takes is enough filmmakers to approve a new system and/or discredit the old system to validate a new system. I'm sure that nothing has changed at the MPAA due to TFINYR because they have no incentive to do so. However, complaining about the system - even via documentary - does nothing to solve what Dick considers a problem.