With all due respect to Law2Doc, I don't think that getting both degrees together is a bad idea, even if the career path is not clearly defined. More than anything else, I think that the combination of the two degrees creates tremendous possibilities in the settings Law2Doc identifies --- academia (for example, Ellen Clayton at Vanderbilt with dual appointments), government/policymaking (for example, David Kessler, former FDA Commissioner), or practicing either law (actually on either side there are a number of MD/JD plaintiffs malpractice attorneys too) or medicine. There are also a growing number of medical-legal collaborations (for example, the Family Advocacy Program at BU --
http://www.familyadvocacyprogram.org/ or the Pediatric Health Advocacy Clinic at UM) where legal services are provided in a medical setting in the hope of improving health outcomes for underserved populations. This is not to say that it is realistic to be able to keep up with developments in both fields enough to practice both (although apparently some extraordinary people do this...), but rather that MD/JDs can serve to foster these sorts of programs. This is why Im seeking both degrees to be able to practice medicine in an underserved community with skills to be an effective advocate for my patients. In addition, getting both degrees together can save a year of tuition/expenses (~$50,000).
In response to earlier comments, there are quite a few MD/JD programs now and most schools without a formal program will allow you to obtain both degrees even if no "formal" program exists (I know my school definitely permits this).