I wrote a series of papers on this last year. There are a lot of issues and variables at play here... that all affect the data. I don't have institutional access to the article right now, but some thoughts:
-- There is a standing about two models of care: The Anglo-American model providing for para-professional (EMT/Paramedic) assessment and basic field treatment with rapid extrication to a hospital with a doctor, and the Franco-German model bringing the expertise to the patient (MD/RN on scene, allowing for scene assessment and treatment, delaying transport to a hospital.
There is an argument that the real benefit from HEMS comes from bringing advanced personnel to the scene (APRNs, CCPs, etc), and that the actual rapid extrication (transport) matters less. Perhaps we should be delivering the advanced personnel and equipment via chopper and transporting by ground (with HEMS crew and equipment). I'm not sure I agree.
-- Depending on the region (and it's politics, among other things), HEMS providers have a strong incentive to be dispatched as often as possible (sometimes based on MOI only, for example in MVCs) and transport as often as possible (even if not medically necessary upon arrival). This doesn't support the rationalizations for HEMS, but certainly justifies the existence of the crew and expensive equipment, and the for-profit provider.
-- As far as I know (and as of about a year ago), there were no scientifically founded studies of Cost-Effectiveness.
-- The advantages of HEMS stem for the rare MCIs and large-scale events. They allow patients to receive a high level of care, and depending on management, allow the first (and sickest) patients at a scene to be transported first, and to a distant hospital, spreading the patient load and not overburdening the closest facilities. It's hard to quantify that in a single study.
In a case in Israel in June 2005, HEMS proved it's value following a fairly rural train accident in Revadim, Israel. 289 passengers were injured, and a total of 35 trauma victims were transported by air. The length of time between the first helicopter landing and completion of the air evacuation was 83 minutes. Subsequent journal discussions and analyses hailed this disaster as impressive management of evacuation, particularly the fact patients were evacuated to four Level I trauma centers 40-80 km from the scene—decreasing local burden on adjacent hospitals. The HEMS evacuations that occurred, happened rapidly, HEMS crewmembers triaged alongside ground responders, and crew compositions were adapted to manage the incident appropriately. (citations available upon request).
I have a lot of other sources and discussion points, but my internet is running out. I'll just clarify: I have not yet been proven of the benefits (or greater value) of much of HEMS. It remains the most expensive portion of prehospital care, and providers are more likely to die in the workplace in the country. Are these costs worth it, for a marginal improvement in patient outcomes?