Generally, an MPH that does not fit into the totality of the pattern that your application presents about yourself, will be of little help and could in fact hinder your chances. . Prior to the explosion of postbacc/SMP, MPH was often used by subpar students believing it would somehow enhance their GPA or MCAT. The adcom community quickly caught on and MPH that didnt fit into the students background or pattern was highly suspect. (please note I earned my advanced degree in under a medical sociologist formerly from the Columbia school of public health so I am not trying to disparage MPH). Also, applying to MD prior to completion of the MPH begs the question of motivation and commitment in the eyes of an adcom.
On the flip side, for student with decent GPA and MCAT, like the OP, if the MPH fits into a pattern that you have established, it could be helpful, but certainly a long and inefficient method. For example, many student view only hardcore scientific research as really useful in enhancing an application. Social, psychological, policy, economic, etc research with a medical connection are often overlooked. If you had something in these areas as a undergrad, and then followed with an MPH it would fit the pattern in your application and speak to your motivation and commitment to medicine.
As a side note, for those who have graduated, taking an MPH course while applying (as opposed to being full time program) does help a bit in showing your are still doing something towards medicine. I simply suggest this so if you go to interview you can say something about your what you are doing currently instead of just some job.
As I said however, for most students this isnt true. In the original poster's situation, prepping and taking the MCAT again with a goal of 33 or more would be a more direct enhancement.
In short, only take the MPH if you have an interest in the field and are willing to commit to it. Taking it as a ploy to help you in MD will likely be seen for what it is.