I think you're misreading the comment. The reasoning isn't missing, it's implied by decades of good design principles: simpler is better. A normal developer would intuitively prefer <output>, so the question isn't "Why suggest <output>?" but rather "Why is <output-format> necessary?"

And in the context of LLMs, this isn't just a matter of aesthetics. More verbose tags mean more tokens, and more tokens mean higher costs. It's a perfectly valid and practical piece of feedback.