It's not so much that doing well on a standardized exam means you'll become a better physician. It's more the fact that people who are able to set their abilities in the direction of doing well on exams and getting good grades will be more likely to have the ability to do well in other aspects of learning as well. A high score on the MCAT is much more than just a number. It's an effort to determine how well you can sit down and focus and learn the material. If you can't do well on an exam because you're "not a good test-taker," then how is that going to reflect on you when you're taking a board exam later in residency and are unable to learn the material that will directly affect how you care for patients? If your grades suffer because you went through a personally distressing situation, then how's that going to reflect on your competence when you go through a similar situation while you have to treat patients?

There's an effort here to minimize the importance of the GPA/MCAT, but honestly, it's more wishful thinking and sour grapes on the part of people who simply aren't able to keep up.