I'm not saying I feel like reasoning is for truth-seeking, I'm saying that to some significant extent it has to be - like Eugene_Nier says, even if there's a lot of noise and social posturing involved, on average it has to bottom out in truth somewhere, else why would we evolve to put effort into something worthless? If it was purely about social dominance, why talk at all instead of sticking to fighting/physical displays?

edit: Although I'm not sure how much purely social content can be built on top of a little physical truth - maybe a lot.