My argument is that the essay is potentially the least reflective aspect of the application because it is the only one that an applicant can wholly forge or have someone else to do for him or her.
Even if you do it yourself, the access to resources or coaching or revising for essays is certainly a reflection of SES, just as prep for the ACT/DAT is. So I'm not suggesting essays are more socioeconomically restricting then the ACT. Probably on par. But at least with the ACT you get a reflection of how the student performed. With the essays there is no assurance of that at all.