When I student interview and I see an applicant with plenty of research, I always ask this question. What I'm thinking is: If you loved research in undergrad so much, why don't you want to continue? An acceptable answer is "I don't want to commit to doing research 90% of the time in my career like the MD/PhD program wants me to." or something like that. I'm less thrilled about the time commitment answers, as you're in it for the long haul anyways if you're going into academics.
Quite honestly, since the MD/PhD programs are expanding, getting an MD and doing basic science research is getting rarer. With the graduating debt load of MDs, nobody expects them to go make peanuts doing research. In many cases I try to talk to the applicants to let them know about the current issues in MD vs MD/PhD training when it comes to research.
I've seen a few applicants over the years with great applications who have had a hard time getting into medical school because they were very focused on research and didn't apply MD/PhD. I think this has to do with two factors. First, some adcoms believe that those interested in doing research primarily should not be in MD programs anymore. Second, some applicants have tons of research and very little else on their application. While the MD/PhD program is fine with this, MD programs want to see more diversity. As for the people I know, they sometimes didn't get in at all, and in other cases ended up at lower-tier programs when I thought their MD/PhD application would have put them into the top-tier.
Now, this isn't meant to scare any of you. If there's more on your application than research, don't worry about it. Make sure you have sound reasons for not applying MD/PhD if you get asked that question. If you want to talk about it some more, I'm always happy to. Everyone knows that MD/PhD isn't right for everyone. For the interested, here's an article entitled "The role of medical school admissions committees in the decline of physician-scientists."
http://www.jci.org/cgi/content/full/111/6/765 (Also check out the replies
http://www.jci.org/cgi/eletters/111/6/765--very interesting)