Actually, I wouldn't be surprised if that were true. We had a couple of MD/PhDers who decided to switch over to the MD program in my class. I've heard that Tufts (or BU, can't remember which one) is notorious for people switching out of the MD/PhD program too, apparently there is rumor going around that it is easier to get into MD/PhD and switch out then it is to get into MD there (they have a very financially poor MD/PhD program, they don't pay their students there). So 20-25% as a national avg sounds about right to me. I imagine that it is a smaller percentage at some of the big name academic power houses, but you'd be surprised, once you realize all of your friends are going to be going on without you and be able to graduate 3 yrs earlier then you are, it really tests the resolve of the MD/PhDer to stick with it. Particularly once you learn how much academic physicians make compared to their peers in private practice; I think that a lot of people don't find that out until after they are already in med school.
As for the policy of what MD/PhDer have to do financially, it differs from school to school. Some require you to repay your stipend and tuition with interest right away (really difficult to come up with that large sum without going into some high interest loans sometimes), some just give you a stern look as you turn in your resignation letter. Usually, schools get mad at students that drop out, I don't know if they actually do anything about it though. Like at our school, you have to go explain to the dean why you are dropping out; I'd hate to have to do that unless I had a really, really good explanation. So it's worthwhile to find out what your schools drop out policy is before you enroll, you might be adamant in your decision to pursue MD/PhD right now but so were a lot of other people who later decided to drop out.
