Coincidence! Just like I did yesterday, Gary Gutting noted the misrepresentation of Mercier and Sperber at The Stone. Reason is primarily social and interpersonal--that's the point, not that reason is primarily for winning, rather than for truth.
M&S (see yesterday's post for links) are interested in the function of reason in an evolutionary sense--they want to know which benefit caused reason to exist in the first place. So they're trying to reach way, way back into the mists of time. Part of their case for the argumentative theory is that interpersonal arguing, not solo cogitation, is what reason does best now. Does that make sense? If X does B best now, then is that at least a strong clue that X evolved because it endowed our ancestors with B? I would think so, provided that conditions now are sufficiently like conditions millions of years ago. Conditions could be different. Maybe right now we spend more time yammering, and back then they were men and women of few words. It's kind of hard to imagine cave guys and gals having big debates like we do. We have a lot more time on our hands, what with the division of labor and modern technology and all. So ... I don't know. In any case, I don't think the interest of M&S's article lies entirely in their evolutionary argument. It would be very interesting even if it were just true now that interpersonal arguing is what reason does best. That would support saying that's what reason is for in some loose, but not uninteresting sense.