Economically, it's not very beneficial as well. In general, people who do a PhD after a masters only increase their chances at landing a job by maybe 3-5%, which doesn't really seem worth the money and struggle. Once you do a masters, that's when you've reached the most efficient intersection of money spent and job procured. If you can get a research job with an MPH, why not just get paid well to do it, instead of getting paid a crappy salary for doing your own research - unless you REALLY want to do your own thesis and research and have that sort of drive. I also agree with
Stories in that you might risk being overqualified AND still have less job experience than people who went into the job field right after getting an MPH, both factors that put you closer to the bottom of the list.
That being said, a PhD in a science area is much more valuable on the market than a PhD in most other areas of academics. Of course, that might be a matter of opinion.
For a good discussion on the value of getting a PhD:
http://www.economist.com/node/17723223
I would read the comments section for some really good food for thought.