http://www.newyorker.com/archive/2005/10/10/051010crat_atlar...
"The difficult part was coming up with a way of keeping Jews out, because as a group they were academically superior to everyone else. Lowell’s [Harvard prez, mid-20s] first idea - a quota limiting Jews to fifteen per cent of the student body - was roundly criticized. Lowell tried restricting the number of scholarships given to Jewish students, and made an effort to bring in students from public schools in the West, where there were fewer Jews. Neither strategy worked. Finally, Lowell-and his counterparts at Yale and Princeton-realized that if a definition of merit based on academic prowess was leading to the wrong kind of student, the solution was to change the definition of merit....
...The admissions office at Harvard became much more interested in the details of an applicant’s personal life. Lowell told his admissions officers to elicit information about the "character" of candidates from "persons who know the applicants well," and so the letter of reference became mandatory. Harvard started asking applicants to provide a photograph. Candidates had to write personal essays, demonstrating their aptitude for leadership, and list their extracurricular activities. "Starting in the fall of 1922," Karabel writes, "applicants were required to answer questions on ‘Race and Color,’ ‘Religious Preference,’ ‘Maiden Name of Mother,’ ‘Birthplace of Father,’ and ‘What change, if any, has been made since birth in your own name or that of your father? (Explain fully).’ "
As a Princeton admission officer for 2 years, I read probably 5,000 essays. A few were memorable. Most were forgotten as soon as I finished scribbling out my summary. The point of the essay was to write something that would leave a mark on my brain so I might take an interest in your application later around the committee table during deliberations. Some kids were great at this, others not so much.
That said, I don't recall admitting a single student based on the strength of their essays. They were there to add flavor and color. If you didn't have "the credentials" you weren't going to write your way around it.