Here is an interesting recent case out of the Illinois Appellate Court holding that an emergency-room physician who rode in an ambulance transporting a patient between hospitals is immune from malpractice actions under a state law primarily aimed at protecting paramedics. The court found that EMS medical directors — who are immune under the Emergency Medical Services Act — may engage doctors to ride in an ambulance when an emergency situation requires more advanced medical skills that paramedics don’t have and that "[a]n opposite outcome would serve to discourage EMS [m]edical [d]irectors from utilizing medical personnel with advanced training during such emergencies and non-emergency transports, which would defeat the very purpose behind the [a]ct and its immunity provision.”
I am not sure I agree with this last statement. If anything, the opposite outcome may serve to discourage doctors from agreeing to help EMS personnel because it is the doctors who might risk possible liability, not the EMS directors. But in the end, the policy question is the same. If accompanying the paramedics is optional, then offering immunity is a way to encourage doctors to do so.
You can read the full opinion here.