Elfie, what is the basis for such an authoritative statement, "In 90%+ cases....."
It doesn't square with me. With the applicant pool at T20 schools shifting to folks with non-traditional backgrounds (people who have spend time doing interesting things after UG school) and with schools expressed interest in diversifying their classes in terms of talent and socio-economic backgrounds, the essays are a key source of information.
It's really the only opportunity for an applicant to frame the information that Lizzie M cites above. Applicants should devote appropriate care to them, imo. I think they often make a difference in who schools choose to interview.