Haha, I'm 2 coronas deep, you're about to kill my buzz.
1. How can you tell that one applicant is better than another? Higher scores in the pre-req courses, higher MCAT? AAMC publishes the data for MCAT and GPA, but the subjective criteria, such as passion and purpose, are conveniently left out. To quote Dean Zic at Vanderbilt, "an MCAT and a high gpa don't determine the quality of physician that you're going to be." Therefore, schools really do look at the application holistically to pick the applicants that they truly feel will be the best physicians and enhance the learning experience of the other students.
2. I am certain that UCSF could have a class full of students who average 3.9/36+, ie: Wash U. But UCSF doesn't because they want a diverse class: culturally, economically, socially, politically etc. They want students with amazing stats, students without amazing stats, students that will go into research, others that will go into primary care, others that will go into health policy... etc.
As far as URM vs. socioeconomic status goes, I think both should be considered, with most of the emphasis placed on experience working with a population and personal obstacles that have been overcome.
Please keep in mind that the health of the underserved community is MUCH more than mere access to healthcare. Rather, it consists of economic stability, education and community leadership. And an individual's contributions to these critical facets of community health can not be enumerated by "stats."
A lot of premeds have the anecdotal story of URM X who was rich and benefitted from AA. Well for me, and many other URMs, the story is completely opposite.