The general idea behind the combined MD/PhD program is to train people who will go on to use basic science research to study clinical problems. Although this is what biological PhD scientists do (their research, on some level, deals with a physiological or pathological process), MDs have a different perspective.
For instance, you can study viral pathogenesis and learn about all kinds of mechanisms that will contribute to a better understanding of viral pathogenesis. But, MD/PhDs have a better sense of how to link current knowledge with finding new therapies or diagnostic tests. Or, at least, it is easier to ask "What would help me better treat this patient?" and develop hypotheses based on patients.
However, with both degrees, you have the option to do whatever you want. Some MD/PhDs never do a residency and only do basic science research. Some do a residency and never do research again. Some do clinical research (drug trials). It is up to you. Many run a basic science lab while having some clinical duties like seeing patients on certain days of the week, or acting as the attending on a clinical service for a week or month at a time. It all depends on the specialty you choose and the department in which you work.
However, it is a long, tough road to go if you know up front that you do not want to do or like doing basic research. The dual degree is not worth doing just to make yourself more competitive later.
