It's an awkward point. I think it's much like ADHD - I have seen and interacted with children who have ADHD who definitely have it. There is definitely something very different about the way these children's minds work. However, I have also seen and interacted with children who have a diagnosis of ADHD (often just unconfirmed by their GP, or even their parents insisting "he's ADHD, honest") who are frankly just little brats in need of some consistent parenting and being told "no" once in a while. Similarly, I wouldn't presume to say that BPD "doesn't exist"... but someone demonstrating a few symptoms could very well just be playing for a bit of attention, and giving the people who are really suffering with it a bad name.