We used to be members when we first opened our doors because we felt it was a great way to network. Their PMA University that they hold during BEA is illuminating for the newer presses because you're meeting professionals in all different aspects of the industry. I've made good friends with people I met there.
They offer booth space at BEA, which is helpful to the newer press who wants to have a presence and doesn't have the money to get their own booth. They also offer a lot of really silly programs that do nothing but separate you from your money - publishers who don't have distribution and such. However, I found their magazine to be helpful in the early years as well.
But in no way should it be confused as bragging rights. Anyone can join if they have the bucks to pay the yearly fees. Like Jim said, PA is a member. In short, they're good for networking, but we never found that they helped us sell books - which is far more important.