What does an MD/PhD mean to me? Granted, my answer is biased in that my goal is to do 80-90% basic science research in the future.
1. Opportunity to learn the vast array of knowledge that medicine has to offer. This enables the investigator to ask interesting questions that non-MD investigators may not come up with as naturally.
2. Get excellent training as a scientist (but you can get this through a straight PhD program). MD training doesn't train you to be a scientist.
3. Staying ahead in this day and age of credential inflation. Before, having a PhD was perfectly sufficient to get a tenure-track faculty position. But now, many people are getting PhDs making the job market more competitive. The average # of years required to complete a postdoc and compete for tenure-track positions has been increasing. So the MD helps out tremendously. If you can go to a hospital and offer valuable clinical service, even though it is part time, the institution will clear out some lab space for you to keep you happy.
4. PhD investigators get paid squat. Having the MD and putting in your 10-20% clinical work hours will double your salary. But you won't be making nearly as much as someone who does solely clinical work. But c'mon, let's not ask for too much. And plus, if you salary doubles, you live that much more comfortably
5. If you don't get tenure, you can still keep your clinical appointment (safety net phenomenon) and not get automatically booted from your institution.
6. Having an MD will increase the availability of grant funding for which you can apply (i.e., I think there are grants that are only available to those with an MD after their name). Hence, more grants mean more money for the institution meaning increased likelihood that you WILL get tenure.
Can't think of anymore...I wanted to get to 10
Anyways, that's my 2 cents.