a doctor is like a mercedes
"You may not need one, but you want one."
Sorry, I love that commercial...
More to the point.
What a physician brings to the table is knowledge, experience, and the unrestrained ability to make a decision.
Emergency "protocols" or scope of practice in various nations of the world are based off of epidemiology. They are treatments that will help or at least do no harm (so we thought) the most common afflictions.
Whether or not we need a physician on an ambulance or helicopter depends entirely on the manner in which they function. If all they are doing is following a script (aka protocol) then no, they are not needed and it is a tremendous waste of money and time.
Most in the US seem to think a doctor is beholden to an institution with labs and xrays and all manner of technological might. It is simply not true.
A doctor determines what is going on with a patient by collecting data. Sometimes it is possible to collect lots of data, like in an academic hospital on a stable patient. Sometimes the data available is more limited, like on an unstable medical/trauma patient during the monsoon in Bangladesh.
Now all this data collection and the word we all love, "diagnosis," may give the impression that there is a lot of focus on learning what is wrong and not so much on fixing it. It is a common blue collar mistake to think what you see is all there is to it. Once you know what is wrong, for a physician, and even for more senior medical students,
what to do about it is intuitive. If a patient is bleeding, then the bleeding must be stopped. If the patient is in heart failure, then after the acute symptoms are treated, something must be done to make the heart pump more efficently, as well as reduce modifiable factors which advance the disease. We have all manner of things available, medications, surgeries, and various technologies with which to work with.
In the absense of all of this stuff a physician is still not at a loss. The founding knowledge of science allows things to be "made up" or "scrounged" in order to help a patient. For example, I know that lidocaine does not work on intact skin, and I also know that many OTC burn oinments contain 0.5%. So I can reduce pain without a vial of stuff and a needle. I can calculate the nutritional values and compositions of various foods to reduce/increase the amounts of a specific amino acids or trace elements to bring a patient relief in various disease states.
Whether it is a basic talking about a medic, or a medic talking about a nurse, or a nurse speaking of a physician. The world is filled with people who think the value of these providers is in psychomotor skills or the depth of protocol they follow. What happens when a patient doesn't fit a protocol? How do you know if a patient doesn't?
Practice makes perfect. My textbook for history and physical exam is not only written at a higher educational level, but it has far more findings than most EMT books have pages. In orderto even make use of it you need detailed knowledge of several basic sciences. From just looking at patient I can gather far more information than most in EMS. Data collection at its finest. I listen to heart tones on every patient i see or have seen for the last 4 years. How often do EMS providers? Will the information gained benefit an EMS provider? Not always, but it always benefits me, even if it is normal.
Whether a doctor is econmical or beneficial is dependant on how the system utilizes her, not on the considerable abilities the physician brings. In one system a physician might be the greatest asset in the world. In systems like urban USA, it is a tremendous waste that is totally unnecessary.
What good is sending a physician to a hospital where they cannot practice because they don't have priviliges or force the physician to simply follow a protocol no matter what the patient or finding?
On the other hand, the value of one who can operate to the fullest of independance can keep people out of hospitals, reducing healthcare costs and providing better care. As well, when you have a really complex patient, simple training will not do. If it were somebody special to you, you might want the person who could handle it, rather than a person who couldn't that was cheaper.