I'm back in school for an MPH as part of Prev Med training after practicing for a while and I love it. There are a number of med students in my classes, as well as fellows straight out from residency, and fellows who'd practiced for a while, other types of clinicians as as well as non-clinicians. First, the MPH and my salary are paid for by someone else, since it's part of the residency program. Secondly, having practiced in real life gives me a different perspective on the subject material. For instance, I've actually experienced capitation and the different ways to pressure doctors and their support staff, or where an MRI fits in care of non-operative knee pain, or how to arrange care for outpatients who don't have insurance. I've also been practicing long enough to discern between what classes are good for me professionally (bio and epi) and what classes I find interesting to me personally (environmental engineering and nutrition), and which really are more important (the latter, rather than the former.)
Having said that, taking classes in Epi and Biostat are good for people who want to do research some day in any clinical specialty. If you take them before med school, you may find that not using those skills allow them to dissipate over 2/3/4 years. It's also hard to find interesting masters project if you don't have a lot of experience.
I think it depends on why you want the MPH.
Trix