Integration of cirriculum, learning how to think about medical and scientific problems off the bat will make you better able to do this in your career. Another answer is that the integration will reduce the amount of time training versus getting the two separately.
Once I was pushed further and I ran out of things to say. So I told him the truth, "Free medical school and stipend. I mean, if you come out of med school $200k in debt, that's alot of pressure not to go into research." The interviewer seemed to like the answer, though I was promptly rejected from that school. haha.