I don't think it's possible to answer this question. MD/PhDs are distributed across all specialties and are doing all kinds of work. Some do very clinically oriented research, some do translational lab stuff, others are basic scientists barely concerned with applications.
Non-combined degree physician scientists cover the same range (i.e., there are MDs who do very basic science, and probably just as many as there are MD/PhD). You might be tempted to ask them, Why didn't you get your PhD? How can you do this work without one? I will let you ponder this little paradox at your leisure. [Part of the solution to the paradox is that many of these people came up before MD/PhD was a well-established pathway.]
You want to know what kind of research MD/PhDs are doing? Pick any research-oriented medical school web site, pick any department (clinical OR basic science), find the faculty roster and browse.