That could be just their way of pressuring you to really explain why you want and need an MD/PhD. I think the schools must be under some pressure to select the right MD/PhD candidates who are not just looking for a freebie to years of medical school or whatever you get if you decide to drop out of the PhD end. It's not too hard to say research research research and then say you changed your mind if there is no consequence.
Yea, it was a very strange interview. It wasn't like my interviewer said I think you don't need a PhD to do research why do you think you need one? I don't want to get too in detail about the specifics but my interviewer just told me a really long anecdote about how his friend did an MD/PhD and wasted so many extra years in school and then now only does clinical work while he finished his MD faster and now does better research than his friend. Didn't quite know what to say to that and there wasn't much of a question in it either. I guess I am actually pretty happy to have a second chance and hopefully get an interviewer I connect with.