Even doctors don't have an obligation to serve the patient's financial interest. I used to work for a Chicago-based company, and they offered Blue Cross / Blue Shield of Illinois. In California, where I live, these are two separate insurers. The doctor's office got to choose which run to my treatments through.

After finding that they were doing it in a way that was advantageous to them and disadvantageous to me, I asked a friend who works in medical ethics and billing whether doctors have a duty to serve the client's best financial interest. I was surprised the answer was no because lawyers (my former career) certainly are required to do so.