We can only become "post-racial" by acknowledging the role that race plays in our society. Otherwise, too much systemic and subconscious discrimination will get swept under the rug. Too many people who claim to want to be blind to race are either naive or willfully wanting to perpetuate conditions that are not equal based on the long history of discrimination in our society.
And Harvard's admissions aren't just about selecting the best collection of individual students. It's about selecting the best cohort of students. It's not just teachers that educate students, it's other classmates and a diversity of perspectives benefits all students. A good chef doesn't just choose all the most expensive ingredients, they try to balance flavors. Admissions departments should do the same, since their goal is to maximize the post-graduation success of the class they assemble, not to capture as much pre-admission achievement as possible.
For me, college was a time when I broadened my perspectives greatly. I can remember a discussion on, somewhat ironically, affirmative action in admissions, in which an African-American student said he didn't support it. His reasoning, which was eye opening to me at the time, was that he had no way of knowing whether he actually belonged there and worried that people looked at the color of his skin and assumed he got into school because of his race and not his intelligence. Up until that moment, I'd always looked at affirmative action as benefiting minorities at the expense of more deserving students. But I'd never consider the emotional toll it took on minorities. And that moment also drove home the fact that I'd never have gotten to hear that perspective if he hadn't been in that discussion group and, therefore, my education had been improved by his presence. After I made those points, it seemed to hit him that his presence there was valuable to the rest of us and not just the more limited way he'd seen it before. It was a bit of a kumbaya moment and several students remarked that their views of affirmative action had become more nuanced and (pardon the pun) less black and white.
The one area where I do agree with you is that I think racial categories often lack the nuance necessary to have that broadness of perspective in a college class. Lumping Southeast Asians, East Asians and South Asians together, for example, feels wrong to me. Similarly, lumping American caucasians with Russians, Scandanavian or Spanish people feels equally reductionist. Within the overly-broad racial categories, there's a lot of diversity that isn't necessarily recognized.