I'm not really advancing the proposition to only teach knowledge that will be used. I was mainly trying to point out that (1) stated or implicit goals (according to whomever) for the educational "system" largely contain their own distortions and contradictions, and (2) learning that can be used tends to be more robust. As simple as that is, it does seem to be often ignored.
On the main point of your original posting, I haven't yet read Mark Saul's rebuttal to Liping Ma, but I do tend to sympathize with her viewpoint. I'm not one to disagree with teaching arithmetic to all children.