There are two different arguments advancing. I can understand BigTex's reasons based on proportion, since there's little controversy. However, I would like to add, there is probably a critical number that a minority group has to achieve to overcome these said cultural challenges. A policy based on percentage representation may not achieve the intended effects for SMALL minority groups.
Mobel's argument on the other hand "Historical crushing socioeconomic disadvantage", does not justify why a white person whose family tree contains few if any college graduates is not considered, nor does it justify why a URM from a wealthy family would be considered. If anything, it supports the original question of exclusion.