We really don't see enough of Chopper being a doctor, beyond occasionally following Zoro around fretting about his bandages. I think, on a ship full of teenagers, there would be a lot of awkward doctorly lectures. And Luffy would keep going back for more, because Chopper would pat him on the head every time (even if he had to stand on a stool to do it) and would give him a lollipop. XD