I have actually been advised against going to a public policy phd program from my economics professors and recommendation writers. The reason was because they viewed it as vague and harder to employ in terms of finding placements in academia. It seems that public policy schools tend to hire candidates who have graduate degrees in specific disciplines (economics, sociology, political science, etc.) than public policy. I was also told by a couple of people at the IMF, IFPRI, etc. that they prefer specific disciplines when hiring, but that if you do a great dissertation that it will be less of an obstacle. It seems that public policy phds are viewed with some skepticism (although I'm sure this depends on the public policy program a candidate was in).