After having students try to write analyses of various kinds of nonfiction, I can see the attraction in teaching them fiction that doesn’t have a specific message it’s trying to impart, primarily because a lot of students simply don’t have sufficient background knowledge to add anything to most of the nonfiction they read. They don’t read nonfiction very carefully, which means they have trouble making any statements other than bald assertion and frequently saying things that be countered through appeals to the text itself.

This could be simply because they haven’t read enough nonfiction. I don’t know what sort of environment you teach in, but I suspect that teachers of lower grades focus almost entirely on fiction, so when you introduce students to nonfiction, it forces them to think in new ways. Even if they don’t have any original ideas other than “bald assertions,” the very act of reading nonfiction should stretch their minds.

It sounds like you’re saying, “They don’t have much experience with nonfiction, so there’s no point trying to give them experience with nonfiction.” Yes, their responses to writings are non well-formed, but the only way to write better is to practice.

I may be off base or missing something, as I haven’t ever taught an English class, but those are my thoughts.