Well... I give the first person a bit more slack in that situation. Stating a claim can be interesting, even with no substantiation. (It isn't always, but it can be.) But a rebuttal that just says "no, it's not" is, in my view, considerably less useful or interesting or whatever. I expect more from a rebuttal than I do from a statement of an initial position - partly because not all statements of an initial position will get a rebuttal, but every rebuttal is in reply to a stated position.
