I consider engaging and consuming forms of art akin to play. At first sight to spend lots of time playing might be seen as a waste of time but the fact that a child prevented from playing would fail to develop properly says something about its purpose. Basically I don't agree that those subjects are purposeless.
I think the other poster was suggesting that students will perceive abstract symbolic manipulation as useless unless they're taught why it isn't useless, probably via application problems. Considering that the majority of students will never have a genuine interest in math for its own sake, I think that textbooks and curricula designed for public education should include high doses of application problems, with enough theoretical problems to interest the true mathematicians and verify the students' understanding of the abstract concepts.
Or you can teach a generation of students that if isn't 'practical' then it shouldn't be done and certainly isn't worth any effort or struggle.
Hopefully then, we can eliminate poetry, music, art, literature, and p.e. We have practical problems to solve!