Here's an Associated Press Article about the development of the PG-13 rating and how it became the "preferred" rating for movies. The article is relentlessly upbeat, so it doesn't mention the current problem with the PG-13 rating, which is that it now has a completely different purpose from its original purpose. The original idea was that the PG-13 rating was a sort of harder version of PG; it was to be given to films that were basically appropriate for kids but had scenes that might be too violent or horrific for very young kids, like Gremlins or Indiana Jones and the Temple of Doom. But in recent years, because Hollywood is so keen to reach teenagers, PG-13 has become, in effect, a modified R: it's what you get when you make an "adult" film but leave out a few things in order to make it mild enough for PG. Hence you get films whose subject-matter and style seems to demand an R, but which chop out material at the last minute in order to get a PG-13 and bring in the teens. The ultimate absurdity is Alien vs. Predator: a PG-13 movie based on two R-rated movies.
I actually like the idea of making "adult" movies for squeamish adults -- that is, movies that deal with adult subject-matter but don't have a lot of swearing, explicit violence, nudity. I suspect that "squeamish adults" are another one of the markets that Hollywood hasn't figured out how to reach: I've known people, intelligent but squeamish people, who don't want to see infantile movies but who don't have the stomach for the violence or swearing of a Scorsese film. But to make a movie for those tastes, you can't just write an "R" script and then chop out all the naughty bits; you have to approach the material in a particular way, to make sure that the approach you choose will not require the things that create an R rating. A lot of PG-13 movies essentially take an R approach and then compromise themselves for the sake of that rating. That provides nothing for the squeamish adults, who are still turned off by the R approach (with or without the explicit stuff), it infuriates those who prefer the R approach, and it doesn't really please anybody, not even the teenagers.
Note # 1: By advocting "adult movies for squeamish adults" I am not saying that I think all or even many movies should be like that, just that there are some viewers out there who enjoy a crime drama from the '40s more than a recent crime drama, precisely because the '40s crime drama tells an adult story without explicit content; and it would be interesting to see some movies made for those particular tastes (which is not to say that there's anything wrong with explicit content).
Note # 2: There's a South Park episode with a running series of fake trailers for bad Rob Schneider comedies; at the end of each one, the announcer adds "Rated PG-13," in the most scornful tone imaginable. Sort of sums up what the PG-13 rating has become: not a warning that a movie is rougher and tougher than it looks (which was the point about Gremlins) a signal that a movie doesn't have the balls to be rough and tough enough.