Diversity. Not in the racial sense, but in the "variety of people" sense. If you only use one admission factor, than all your students will be good at that factor. You get a school full of good test takers, to the exclusion of good musicians, or athletes, or whatever other skills are valued.
Ok, but I got into Stanford based on test-taking. Got 99% in math and English parts of the GRE. Stanford had a thing for that, back then.
My grades were not great because of ADD - didn't concentrate in HS or even my first 4 years very much, homework was spotty or absent, but boy could I take a test!
