There is nothing wrong with using a Dr. name in the ps. However, once used, in all likelihood you will end up with a ps that is peppered with the name Dr. XYZ. The ps then starts to look more like a testimonial to Dr. XYZ, which Dr.XYZ probably would not mind showcasing in his waiting room. Unfortunately, the ps is about you, the applicant, not about how great Dr.XYZ is in dealing with patient or how good a clinician he may happen to be. And voila we have a ps that looks like this post.