I agree with the overall thought of your comment, and usually agree with all your posts. And again, I do see what you are saying, but I think a PS can definitely include why you want to be a doc, as well as qualities of why youd be a good doc.
In fact, as I stated earlier, I think the main theme of a PS should be why do you want to go into medicine and why would you make a good doc.
HOWEVER, like you emphasize cabinbuilder, if you can express those ideas by means of interesting stories and unique experiences, all the better. In fact, you almost MUST be smooth and even a bit subtle with how you express why youd be a good doc. You can't say, 'id be a good doc because...' but youd have to share a story where you showed empathy, determination, commitment, understanding of people and/or medicine, etc. The readers are intelligent to form their own conclusion that those traits would make a good physician.
So, hopefully that helps. There are several types of PS's out there. If you have an amazing story (like cabinbuilder...read her PS sometime, its epic!). But there are other strategies and forms a PS can take as well. Just make it interesting, and on topic, and polished and you should be just fine.