I think the common mistake that most students make is that having an MBA will make their private practice a better business given the newfound knowledge of business know how. Many good practices will either outsource their financial dealings to a consultant, or have their doctors take a few extra classes in business management to increase their know how. I dont think an extra degree will help too much when it comes to getting into medical school unless you use it for several years, and even then it wont help improve a weak application. Usually, the MD/MBAs ive worked for (one this summer in fact), are in the business side of medicine- that is they work in medical advertising, pharmaceutical type of businesses, and rarely do I see practicing MD/MBAs as a healthcare provider. Unless you demonstrate some strong connection to this through your personal statement/experience, an MBA will most likely not carry too much benefit to your application.
What you will really have to gather is why you didnt get into medical school the first time. If the reason points to a poor UG GPA and BCPM, taking an MBA wont really address those issues, and thus your application wont be improved in the areas in which they are the weakest. Taking on an MBA certainly wont address poor science grades, no matter where you take it. Granted, a grad GPA has been heard to be less "weighty" when it comes to adcom decisions but at the very least, you are addressing those very issues one way or another in the area in which you are the weakest. You are, after all, taking classes taught by PhDs and MDs in material that aren't normally found in an UG text book.