In an ideal use case, one employing sarcasm would take the other's point to a logical conclusion, but to do it in such a way that the hidden incongruity is exposed.
The problem is that using sarcasm assumes that the opposition (and it's always an opposition; sarcasm is offensive and antagonistic) is blind to the faults of their argument, and that you're teaching them something they didn't know. This can backfire if the incongruity just isn't so, and the opposition would have been easily able to explain it to you, had you used a normal communication mode. By using a sarcastic tone, you're creating obstacles for the opposition to normally argue with you.
It is possible, and in fact advisable, to use "reductio ad absurdum" without a sarcastic tone, because the absurdity should be able to stand up for itself.
Sarcasm about one's own feelings ("I'm reeeeally enthusiastic about this!") does not employ reductio ad absurdum, it's just obnoxiousness for the sake of obnoxiousness.
The problem is that using sarcasm assumes that the opposition (and it's always an opposition; sarcasm is offensive and antagonistic) is blind to the faults of their argument, and that you're teaching them something they didn't know.
I think you're ignoring the potential for friendly antagonism, here. Both good-natured ribbing and cruelty can employ sarcasm.
This is my first attempt at starting a casual conversation on LW where people don't have to worry about winning or losing points, and can just relax and have social fun together.
So, Big Bang Theory. That series got me wondering. It seems to be about "geeks", and not the basement-dwelling variety either; they're highly successful and accomplished professionals, each in their own field. One of them has been an astronaut, even. And yet, everything they ever accomplish amounts to absolutely nothing in terms of social recognition or even in terms of personal happiness. And the thing is, it doesn't even get better for their "normal" counterparts, who are just as miserable and petty.
Consider, then; how would being rationalists would affect the characters on this show? The writing of the show relies a lot on laughing at people rather than with them; would rationalist characters subvert that? And how would that rationalist outlook express itself given their personalities? (After all, notice how amazingly different from each other Yudkowsky, Hanson, and Alicorn are, just to name a few; they emphasize rather different things, and take different approaches to both truth-testing and problem-solving).
Note: this discussion does not need to be about rationalism. It can be a casual, normal discussion about the series. Relax and enjoy yourselves.
But the reason I brought up that series is that its characters are excellent examples of high intelligence hampered by immense irrationality. The apex of this is represented by Dr. Sheldon Cooper, who is, essentially, a complete fundamentalist over every single thing in his life; he applies this attitude to everything, right down to people's favorite flavor of pudding: Raj is "axiomatically wrong" to prefer tapioca, because the best pudding is chocolate. Period. This attitude makes him a far, far worse scientist than he thinks, as he refuses to even consider any criticism of his methods or results.