I really don't know much about MPH programs, but I would expect opportunities for people with that qualification. Public health seems to be a pretty diverse field. I could be wrong, but personally I interpret an MPH as being a mix of hard and soft sciences, so while I can see the sensibility of that in some scenarios (particularly larger scale work comes to mind; you know, epidemiology focusing on the bigger picture, bioinformatics, other things I don't understand), I'd probably expect more PhDs on the nitty gritty lab work side of things, which is where my interest lies.