Why I thought it was justified:
He wanted to improve his son's general English skills. The book teaches how to pick out evidence, how to sound smart whilst quoting it, and the structure in which you should do that in. Whether it's language analysis or text response, those are vital skills that anyone needs to know.
Odds are 2/3 of those study designs will still remain in 8 years. The only possible change they could make is moving to persuasive tasks, and/or or removing context. So even if you weren't to touch the book for that long, it's still probably good. That's assuming you only want to use it in year 12, in all honesty Text Response and Context are things which you get started on super early. I got it in year 7-10, so it might not be that long of a wait.