Advantages? The variability in quality of PhDs is high so it is harder to generalize about them. The absolute smartest people I have ever met where PhDs (or PhDs/MDs), but there are some lackluster PhDs out there too. With the PharmD there's a more uniform minimum standard.
I'd say you could look at the combo in one of two ways: Either it represents a perseverant, detail-oriented, literature searching PharmD who gives a pretty good oral presentation. Or, a clinically-minded, broad picture aware, sharp PhD (I think pharmacisty demands "sharpness" – quick thinking multitaskers, as opposed to a plodding genius). I'd use as many of those words above as possible on a job interview.
What do people with that combo do? I'm a PhD/RPh (BS only) so if you figure it out please let me know. Currently, I want to get into regulatory in a pharm company, mostly because I know that those jobs are filled by PharmDs or PhDs, so I think my dual-training will be helpful there.
However, I also found (from internet job searches) that most of the jobs in regulatory for PhDs require 3, 5, 15+ years experience. I have none, so I just started a temp job in medical communications at a big pharm company. So far I've been very happy.
Having a pharmD and PhD and just being a retail pharmacists is my fear about the dual degree. That is why I think I'm moving away from the idea. It may be more practical to do a residency or fellowship afterwards. It seems like the dual degree may be just an easier way to get into academia vs a PhD or PharmD alone.
I'm also considering a PharmD/PhD. A pharmacist that I know thinks it is a crazy idea. Her advice was just to do a fellowship once I finish the PharmD. However, I've looked at a few fellowship programs, and basically it looks like you'll end up doing two or three years of research for very little money. I figure if I'm going to do three years of research for a graduate student-level salary, I may as well get the extra credentials at the end.