I'd say it just really depends on what career tragectory you'd like. Having "just" an MD is going to open most doors in your career. And, if you want to "just" practice, that is going to be sufficient. Most combined degree folks have career goals that might not be simply "med school --> residency --> fellowship --> practice --> retire".
For instance - want to do academic medicine and lots of research? MD/PhD gives you a stronger research background (but plenty of MDs do good research without it).
What to do clinical research? MD/MPH may give you more background for this.
Want to learn more about developing public health programming, health education, disease control and outbreaks, global health, etc. - MPH/MPH/MPH. Also, for some jobs (faculty in public health schools or medical directors of depts of health, you generally need a PhD alone or an MD/MPH).
What to apply business models do your private practice office, or working in medical administration for a hospital or HMO? MD/MBA
Health policy? Liability law? Government? MD/JD
(and so on - you get the picture). One of the best things about medicine though is that there are so many career options when you are done.
