I worked with MD's for about five years as a research associate. I remember asking them about the brutally long hours that interns were forced to work - at that time 36 hour shifts straight were not at all unusual. The response I got from these very intelligent men was that if they had to do it, then the interns should do it too. Seriously. There was some muttering about preparing new doctors for emergency situations where they might have to tend to the sick and wounded at all hours. I suppose that has some validity. I think it's more a matter of testing the new doctor's mettle - if they can maintain composure and not go postal during a 36 hour shift, then perhaps they are sane enough to be trusted with countless persons' lives.
But I think there must be better ways to determine that.