Poverty of aspiration? quite funny really, just don't see this as connected to this very detailed grammatical learning (it is beyond what is currently in Level 6 SPAG). I am actually behind teaching grammar as part of a number of aspects of English, but it is also true that having not been taught any grammar, I am extremely successful academically, and so its value for an entire cohort is just not clear to me.
My biggest problem at university level teaching is not the students' poor grammar, it is the poverty of being unable to think for themselves. Many of my students think learning is repeating what you have been taught for the purposes of testing and rewriting words they find on the internet onto some different pages and handing them in. I blame the testing (and retesting) regime entirely for this. I also have quite a few dyslexic students, who have poor spelling, but are clever novel thinkers and I value this more highly than nicely written regurgitated material.
It depends what you want- I don't see why you couldn't teach this stuff, but as for obsessing about making every child pass this quite difficult test, thereby managing to sow failure in some and an inability to think for yourself in others, no thanks.