because in the first instance, all you're saying is "prove it", implying that the prior reasoning was deficient without specifying the insufficiency. that's vague, lazy, and ultimately unproductive/stifling. at least point out the deficiency, thus furthering the discussion.

the counter is not a non-furthering "citation-free expression of belief", but rather a rationale, a chain of observations leading to a reasonable conclusion, which can largely stand on its own and also furthers the discussion.