<p>All joint degrees have very specific uses.</p>
<p>MPH: Sometimes (always?) has the advantage of shaving a year off of it for MD's. Used mostly for rural health practitioners who are responsible for considering the health of their communities as a whole, not just their individual patients. Prominent example of an MD/MPH: Julie Gerberding, head of the CDC, widely praised for her response to the avian flu discussion. (With a past that has had its share of controversy relating to AIDS.)</p>
<p>MBA: Has the obvious use of being related to hospital administration and health care insurance administration. Also has the less obvious (but, I believe, more important) use of being very related to the leadership roles a physician must take: leading surgical teams, encouraging patients, teaching students, etc. Prominent example: William Winkenweder, head of medical affairs for the DOD (presumably including the VA system).</p>
<p>PhD: Perhaps the most common degree, at least among the physicians I know. Dominant use is for research - but this often includes clinical trials, which is very, very useful. Also helps understand how to read clinical trials and scientific studies, but the PhD is overkill for this purpose. Prominent examples: numerous, including many researchers at any biomedically prestigious university. One interesting example: Mark McClellan, head of Medicare and Medicaid (PhD in Economics).</p>
<p>JD: Probably the most rare combination, estimated at no more than 1,500 people across the US. Dominant use is for lawyers who plan to enter medical malpractice fields, but this is widely considered an absurd overkill. (Most people who have both degrees entered one career and found it unsatisfying, and do not use their degrees together.) Would be well-suited for legal advising (obviously) at hospitals and relating to regulatory issues. Prominent example: David Kessler, former head of the FDA. (Currently Chancellor of UCSF.)</p>
<p>Notice that the examples I've mentioned are all actually quite intuitive choices. Gerberding has to control large disease outbreaks, and so she's an MPH. Winkenwerder has to navigate a large bureaucracy and organize it effectively, and so he has an MBA. McClellan has to analyze the economic consequences of any decision he makes, and how it will impact the country's incentives and health care system, and so he has a PhD in economics. Kessler has to make sure drugs comply with legal standards and that trials have been adhered to in anticipation of lawsuits (i.e. the Vioxx situation), and so he had a JD.</p>