If their only criteria are GPA, test scores, and missionary experience, then they might be passing up some top candidates. But Medicine benefits from people from diverse backgrounds. I’m not a fan of admitting people who are not qualified in order to achieve a diverse student body. But I’m ok with setting lines for qualified candidates and then selecting candidates above those lines based on a variety of different criteria.
As an example, when I applied, I was an out of state resident with a diverse military background. I had commanded hundreds of cadets through a variety of leadership positions at the Air Force Academy. I had spent a summer researching anthrax with the Department of Defense in San Antonio. I spent a summer working in military hospitals in Germany. I captained championship teams in club sports. I graduated in the top 5% of my class. But I’m willing to bet there were a few BYU grads who had better MCAT scores than I did.
The University decides what “diversity” means and they value unique backgrounds and experiences. I don’t think that equals “lesser candidates”. I’ve met plenty of smart people who are terrible physicians. There’s a lot more that goes into the selection process than picking a bunch of candidates with good test scores from the same school.