I would say it's not so clear, and it's open for debate until hospitals open up their books. Yes residents need to be supervised, but one doctor can supervise

I would say it's not so clear, and it's open for debate until hospitals open up their books. Yes residents need to be supervised, but one doctor can supervise many residents seeing many more patients than they could alone. If hospitals didn't have residents they would have to hire more doctors. Also consider that a resident costs less to employ than a nurse and that the hospital bills for all the work residents do. Finally, doctors in teaching hospitals tend to be payed less than doctors in non-teaching hospitals.