If there’s no incentive for productivity, many physicians will go to banker’s hours to the detriment of patients:

The problem with a flat salary is that there’s no incentive to maximize work. Show me a salaried doctor (without a productivity bonus) and I’ll show you one who believes strongly in his right to get home by 5PM, regardless of medical problems coming through the door, or over the phones, that day. This translates into doctors who send acute problems that could be handled in the office to the emergency room to avoid hampering their own schedules, doctors who schedule longer appointments, or block off time to allow extra time for paperwork during the day.