I don't think the only evolutionary purpose of reason is to win arguments, part of the purpose must have been to decide on the best course of action, otherwise we would have evolved to not listen to what anyone else says.
... otherwise we would have evolved to not listen to what anyone else says.
That's a death sentence for a great ape. All the great apes form tribes as one of their primary survival strategies. It could simply be that evolution didn't make ignoring others' arguments an option any more than simply ignoring dominance contests could retain one's status in the pecking order.
I saw this in the Facebook "what's popular" box, so it's apparently being heavily read and forwarded. There's nothing earthshattering for long-time LessWrong readers, but it's a bit interesting and not too bad a condensation of the topic:
A glance at the comments [at the Times], however, seems to indicate that most people are misinterpreting this, and at least one person has said flatly that it's the reason his political opponents don't agree with him.
ETA: Oops, I forgot the most import thing. The article is at http://www.nytimes.com/2011/06/15/arts/people-argue-just-to-win-scholars-assert.html