You know how you get so many E-mails from schools saying why they are the best choice for you and why you should apply. How much of any of them can you recall? Writing about why a school should accept you comes off the same but in reverse. If the school has a good reputation and so on, it doesn't matter, you might apply anyway, if you get a generic E-mail from a school you have never heard of, you will only be interested if they say something about the dinosaurs that roam the law school library.
Point being; to get any advantage out of your PS, you can't be generic. Write something that makes the reader think you are a better candidate than what your application otherwise shows. The number one rule that has been touched on but not explicitly stated in this thread is your PS must be positive. If you write a well-crafted essay about absolutely horrible things that have happened to you, what does that accomplish, does the reader have any better idea of your positive attributes? I think you almost have to write your PS like a used car advertisement, just with English above the 1st grade level.