Just did a quick search on Pub Central and got this hit right of the bat:
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1448039
In the abstract it points out something very important, and unfortunate, regarding the physician-patient relationship that I have heard cited numerous times. Many patients feel like they receive better care from physicians of their own ethnicity. This study has numerous citations supporting that finding. Furthermore, it has been shown that these impressions are generally born out by the facts (i.e., whites receive better healthcare on average than blacks and hispanics).
While the quality of care certainly has a correlation to the percentage of the minority population that lives at or below the poverty level, the patient's impression of care should be independent of that. If a patient doesn't think they are receiving good care, they won't go back to that doctor. They will wait 3 weeks for an appointment with a new doctor who does not know them. Probably they will not feel comfortable with the doctor and be fully honest (as House says, we all lie). That brings us right back to the MCAT.
The MCAT is all about giving the med schools and adcoms another metric to decide who will be the people in society responsible for taking care of the sick. Now, if the med schools only accepted 30+ MCAT and this resulted in few black and hispanic doctors what would happen? The underrepresented minorities would have an even lower standard of real and perceived care. The patients won't be comfortable with their doctors and the patients and society as a whole will suffer.
Bottomline, the med schools have decided that being the smartest guy/girl is an important quality to have in a doctor, but it is not the only one. Someone being educated to become a doctor is about what is best for society, not for that lucky someone. Adcoms have the tricky task of balancing all of these factors to decide who should have the privilege of becoming a doctor.
If you think about it this way than it is clear why the average MCAT scores vary for blacks/hispanics/native americans (underrepresented minority) vs. whites/asians (overrepresented).