I don't think that pit bulls are really a rare breed. I think this is misinformation. I think that rare breed means what it sounds like and has nothing to do with what group the breed falls into.
When I was looking up info on my Bluetick I found that the breed was considered rare probably because it is only recognized by UKC not the CKC or AKC. In canada, I could not find a book on this breed at the bookstore or library.
Here is a link that lists some rare breeds. Pitbull is not on the list.