MD/PhD are medical scientists... they spend a lot of there time doing research.. and for a great price more then the average PhD alone.
However you could very well decide to just be a clinician after the PhD.
yes, there are. obviously, anyone who chooses to pursue an MD/PhD has to figure out his/her own reasons for it, but I would just hate to go through all this training and not have at least some clinical duties. Some people feel differently (there are people in my class who I could quite easily pick out as being that type), and it's definitely possible to only do research. However, there are also plenty of MD/PhDs who do both and do them both very well.
Yes, I know one who worked with my mom in pediatric endocrinology @ Children's Hospital St. Louis and she now works for Abbott doing research.
Many MD/PhD's find that it is EXTREMELY hard to fulfill the role of a PI and clinician, and it pays less.
Anyone that leads a research project is a PI, you can be an MD, PhD, MD/PhD, DPT, whatever letters you want after your name as long as you get the grant