MPH's can be very useful, depending on your interests. In general, they aren't worth the time for a private practice.
But if you are interested in any sort of policy or management roles, then an MPH provides useful further training.
Most docs in the upper levels of international aid organizations, government health agencies and hospitals have an MPH...and this is becoming more true these days than in the past.
Finally, we had a professor tell us (an unapologetically biased MD/MPH) that if you want to help a few people, get an MD. If you want to actually make a dent in disease, get an MPH.
It makes sense...what has saved more lives than every surgery ever performed? - ORT for malaria. If the world simultaneously lost x-rays and immunizations, guess how many MORE people would die in the imm. category? The dramatic positive effects of public health on the morbidity and mortality of societies are endless, and pretty surprising. I've even heard of some research showing that every nation that spends a certain % of their GDP on public health has never had a civil war/rebellion. A direct correlation? Who knows, but it makes you think.
Public health is powerful, but it needs to be your passion. Simply helping individuals - like a typical doc does - is nothing to scoff at.
