Sometimes an MPH is a requirement if you want to do specific work for a government agency, like the CDC. I know for the Air Force, as part of becoming a Flight Surgeon, the first year is getting my MPH at any school of my choosing......and the Air Force pays. Dr. Mom is right---if you want to do epidemiology especially---then an MPH is helpful/necessary. It is a good degree to have because it can offer you some extra opportunities.
I know a lot of schools offer an MPH for free to DO students, and some of my classmates are taking advantage of that. Since I don't want to spread myself too thin, and the Air Force will pay for it later, I'm not doing it at the moment.
Best of luck