For most, an MBA isn't a degree for an entry level job, but permits advancement beyond a certain point in many fields. That is why many MBA schools ideally desire one to have worked (in business) prior to attendance. The typical MBA majors/fields are in finance (including banking), accounting, management (including consulting) and marketing. (Some places offer MBAs in entrepreneurship or international business, but these are usually subsets of those I previously mentioned.) Thus if you pursue any of these fields professionally, you will at some point want/need an MBA.
In medicine, I believe the MBA would be most helpful if you pursue careers in hospital administration or medically-related business consulting -- anyone who went to a school that had a health-related MBA should chime in on this part.