I have written one case report myself. I am second author on 4 others, all written by medical students/undergrads. If it weren't for the pretty pictures, I doubt any of them would have actually been accepted anywhere. I have 5-6 other case reports in varying states of completion/waiting for journals to get back to us. My model is to give students the case, H&P, op notes, pictures, and a copy of the latest JVS opened to the case report section. I then ask them to template a case report based on those things. Their draft is typically 'terrible', but if you have good students, it will have all of the content that they need. I basically re-write it and then we talk about why I changed 90% of the stuff that they wrote. Most of it is lingo or stuff they would have no way of knowing. It is incredibly time consuming and yes, I could write it faster myself. The part that I usually have them hold off on writing is the discussion section, unless I already have other articles for them to read and think about while writing. The first two that I did with someone else I had to throw out their entire discussion and write it myself, so I just write it with them now.
I do it because like SS said it is about mentorship. I enjoy it, even though it can be incredibly painful at time. I do not let students write my actual papers. If one of them was the primary driver on any of them, I'd give them a shot, but it is hard enough for me to be learning medical writing and what journals want without having to teach someone as I go along. They can help with data gather, analysis, statistics, background research, etc. But, I generally don't have them help me write.