Snoopy said:
If you want to put your MBA to use, wait until you have your MD (if you don't plan to complete a residency) or until you've finished residency to put it to use.
While, in general, this is good advice, there's a lot of exceptions to be found. Third (or whatever year the first clinical year is) year is a busy time and you probably won't be able to find time to do much outside the med school. It is, however, (as the last poster mentioned) a great time to head up orgs within the med school and make contacts. For example, I became the head of the student government and, during that year, made sure everyone knew that i was getting the dual degree so that later (when I had the time) whenever anyone in the administration thought of a project that'd be appropriate for a MD/MBA, they automatically thought of me.
Fourth year is a completely different ballgame. I did some consulting work for a pharmaceutical consulting company and managed to make a whole lotta $$ during my fourth year of med school (which is really helping now that I've got no source of income until July 1st!). The flexible schedule of most of my rotations (except for my Sub-I) allowed me time to get plenty of work it in. You really will have time during that last year... but only if you've prepared for it and built (and maintained) your contacts beforehand.
BTW, I like the idea behind your site, mward04 - let me know if you need anything.
As for your question, I think that burnout covers a lot of the people who end up bailing.
However, in terms of motivation, there's some people who honestly feel that they can help the world more as doctors in the business world than in any other way. It depends on what you do. If you turn into a health care consultant nobody is going to be fooled - you want the money. However, a pharmaceutical executive (who could arguably need a MD in addition to his/her MBA) or a VC (who would need the MD to analyze breakthrough technologies) could feel that they're truly helping people.
Necessity is also a coinflip. As theduke noted, you start at the bottum rung no matter what. That being said, you'll move up quicker... and whenever your firm is presenting to a bunch of doctors they WILL have you there - even if only for the automatic legitimacy and respect. So it's by no means essential... but it's not a wasted four years by any stretch.