I am a PharmD MD.
I think that the most accurate description is that given by Kwizard. Most people that continue after a PharmD so so because of the somewhat limited role available to pharmacists. I wanted more autonomy as well as a wider spectrum of opportunities than I felt I could find in pharmacy. I must admit that there is a certain need in my ego to be "the top dog." The funny thing is that I didn't like giving advice to physicians and then have them ignore it, and now I give advice to patients and they ignore it. Just a different group of people ignoring your advice and expertise.
As far as advantages to having both degrees, the most obvious is more training and education. I constantly get asked for drug info from the other docs in my practice.
I don't think there is any advantage to getting into med school with having a PharmD vs a BS. I was on the admissions group in my med school and no matter what people tell you, the most important things are high grades & MCAT scores. They don't care very much what your major is. Many people try to blame their failure in getting a spot on their undergrad major/school when they had <3.5GPA and <70 percentile on the MCAT.
The only real disadvantage to going the PharmD route to med school is the extra two years it takes. That ends up costing about 400K in lost salary for the average physician. That is offset somewhat by the ability to work as a relief pharmacist during med school on holidays and such.
Overall I am very glad that I have both degrees. If I was to do it again, I would probably just do a BS then med school because I might have less gray hair that way.
Just to preach a little, if you want the physicians to be accepting of your advice as a pharmacist, I have a few hints.
1. Before giving unsolicited advice ask yourself if it is really important. If the only thing it will accomplish is to make you look smart, don't. If it will truly improve patient outcome, do.
2. If you see something that could be changed, try not to do it in front of everyone on rounds. Pull the doc aside and bring it up there. If you want to make enemies make them look stupid. Make them look smart and they will be your friend and will seek you out for more advice.