I think that you're mostly correct in your observation that DOs and MDs reach the same outcome following graduation(all, after all, are licensed to practice the same profession);however, if I'm reading you correctly, you're suggesting that different "measuring tapes" be used to assess the readiness and ability of an individual to cope with the rigors of a medical education and practice. Let's analyze the current system:
1)Cumulative grades and science grades(which includes a, for all intents and purposes, standardized list of prerequisites)---->This is the part of the premedical education that most closely resembles the classroom learning that will occur in medical school. Since the prerequisites are pretty much the same for every school, it's a chance for the committee to see how an individual compares to everyone else.
2)Extracurricular/exposure to healthcare---->This is a chance for the committee to assess an individual's selflessness and true desire to be immersed completely in medicine. The logic is that if someone is truly interested in the practice of medicine, they will want to be surrounded by all things healthcare-related all the time, as you most certainly will be as a physician.
3)MCAT---->If the performance in class is a measure of how well you study over time on a focused set of topics, the MCAT tests your ability to not only think simultaneously in several genres, but to do it quickly and precisely. Is a test of your reasoning skills more than your science knowledge, in my opinion. Since they don't have time to break out Tangrams at interviews, this is their only shot to see how you perform with frayed nerves at high speeds. Cruel but likely necessary. Since much of the profession is about collecting data (vital signs, drug levels, blood values, EKG, etc.) and matching it against a knowledge base and coming up with a plan of attack, the MCAT is a cursory predictor of how you'll perform under these conditions.
4)The Interview----> Their chance to make sure you can communicate with others without grunting and drooling. It is, first and foremost, a human enterprise, and they want someone who can communicate with other humans. The only real way to do that is to sit them down in front of them.
I really don't see what else could be done to ensure the quality of the applicants. What I like about DO schools is that they aren't so tunnel-visioned on the numbers that all of the other stuff is overlooked. I believe that many amazing future physicians are turned away from medical schools because of the numbers game. However, I do NOT think that if your numbers are weak that it is a good idea to rely on the fact that a DO school will simply gloss over them and admit you outright because of your extracurriculars and desire. If there's one thing that I've learned through this agonizing process, it's that you CAN change your numbers--if not one, then the other (MCAT/GPA). The applications process is a game, like any other. If you want to have a desirable outcome, then you're going to have to play ball(so to speak) to SOME extent. It's not a PLEASANT game by any means, but it IS a necessary one. If anything, it is a test of your own resolve to pursue, at any cost, the profession of medicine.
I guess what I'm trying to say in this long-winded spiel is that, yes, there's probably little correlation between a physician's MCAT score and their ability to be a fantastic physician, there is HIGH correlation between a premedical student's ability to:
keep all their ducks in a row,
perform well in classes over four years of undergraduate while keeping their GPA in constant consideration,
study hard for the MCAT,
volunteer every week,
work in a healthcare field,
keep up with current events in medicine, and
have the resolve to keep applying, no matter how many times it's required, to get into medical school,
and the quality of the individual entering the school. As the system is set up now, it's an impossibility to just apply to medical school on a whim. Just applying to medical school, whether you realize it or not, is a four year (sometimes longer) process. Not a lot of people have the will to go through a four year application process that will eventually lead to ten more years of school before they can finally get a real job. That's why I admire most premedical students(except the gunners that don't have to work at all through undergrad because their parents pay for everything, down to their groceries) for their resolve. Sorry this post is so long.