I'm a little late in the game, but yes, there's absolutely people who wish to disable themselves in a multitude of ways. It's called BIID. As of yet, the only recognised effective treatment for the intense feelings someone with BIID might feel is indeed obtaining the intended disability.
jillio: I fail to see how GID and BIID are so different. At the end of the day, it's an intense desire to change some part of your physical self as to be alligned with your psychological self. If it takes the form of genital reassignment or an amputation... it is essentially the same. More imporantly, there's absolutely no evidence that intensive psychotherapy relieves a person of the feelings assiociated with BIID- just as it does not with GID.
Please, let's not judge others. I cannot say that they are sick because I've never walked those shoes. I'm betting, neither can you.