In the car riding back, we had a discussion about the stupidity of one of the central conflicts of the story. The princess is being presented to the sons of three noblemen, who will then compete to win her hand in marriage. We discussed how this is a terrible choice: choosing any one angers the others and causes either disunion or warfare. Monarchs don't usually marry their senior/inheriting children to their subordinate nobles, and this is one good reason why.
But what if the monarchy is elective? What if, on the death of king Fergus, the next king will be chosen by the clan leaders? The movie doesn't discuss this possibility, as there really is no place for political nuance in a Disney comedy about a girl's adventure with her mother. But that sort of political structure would make the betrothal tournament much more sensible, as both sides have something to gain from the arrangement. Merida's family gains ties to either the next king (and a young man that succeeds at feats of arms against his fellows is certainly setting himself up as a good candidate for this rough-and-tumble kingship), or someone who might support their clan's candidate's claim to the throne, especially if that claimant is his wife's brother. On the other hands, the winning clan chief gain greater access to the currently reigning king, who could grant them various sorts of favors, and possibly even give his son-in-law advice or political experience that could also benefit his subsequent candidacy for the throne.