I think that the essay portion of the ACT/SAT could used to confirm an applicant’s though processes, writing abilities and level of sophistication in expressing him/herself. I daresay that a kid that writes in a sophisticated manner in a prepared essay, assuming that the work is original, will write better, employ more sophisticated vocabulary, and use more complex grammatical structures when writing extemporaneously than a kid that doesn’t. If there’s a significant mis-match between level of expression, something is obviously wrong.
Adcoms want to hear a kid’s voice, but they also want a well-edited piece of writing. They often encourage kids to show their essays to adults for the adults to proofread. Doing so is not employing a “team approach” as long as the kid is doing the writing and the adults are just making editorial comments and suggestions.