I'd recommend this thread for more info. My short opinion is you shouldn't do it if you think you can pick up the MPH skill set by reading into public health on the side. Do it if you think you need some quantitative skills like epidemiology or biostats, but not if you just want to learn more about public health. That can be done by just reading public health literature and journals in your spare time. It's a lot of extra money (not to mention school, and an MD does minimum 7 years training) and you could use the time to do something more useful for an MD like an internship with WHO or CDC for a year.
