chinaski is much more equipped to answer this and will hopefully weigh in, but:
I don't believe many students do the dual MD/PhD - it's much more common to do a PhD after completing a primary medical degree and working for a few years first. As far as I'm aware combining an academic/teaching career with part-time clinical work is also quite common. Obviously if you want an academic career, doing a PhD is going to open more doors for you than if you have a bare MD.
I believe nowadays it is becoming more and more common for doctors to complete higher degrees and other postgraduate qualifications due to the increasing competitiveness of getting onto a specialty training program, as well as finding a consultant position once one has completed their fellowship (this being most difficult in metro areas). I think chinaski has mentioned before that it is almost unheard of for one to gain a consultant job in a metro area nowadays
without a higher degree to complement their initial fellowship.