You are correct that it doesn't need to be fixed now. However, it would be a big advantage to master language usage/mechanics now. They need to write essays under time constraints for high school magnet assessments next winter. Even in the past year, they have had to knock out essays for scholarship awards and club membership opportunities (NJHS, etc.). It is increasingly evident to me how critical writing skills are at an increasingly younger age.

There is no spelling per se on the SAT Writing but I think correct word usage sometimes comes down to spelling. I never learned grammar rules (preempted by GT classes in middle school) and sometimes lack even the vocabulary to describe usage/mechanic errors. I guess while I have absorbed the patterns without learning the actual rules, I can't help but think that there must be some benefit to actually learning the rules.