 What is at stake in the disagreement over how to conceptualize these ongoing courses of action?  In the example we’re using, the friends’ goals change dramatically from time to time; seen through the lens of rationalization-by-desires, as standardly conceived, the segments of the friendship are only collected into a developing trajectory retrospectively.  The cost is not just that one misses seeing what unifies the activity while it is in progress; often enough, the inability to understand the activity as unified, despite the very different appearances of its segments, undercuts its unity. (Not always: Brewer adduces Augustine as an example of someone who did not understand his life as a search for God until late on, even though that is what it was and had been all along.)  But then, one does not have that dialectical activity – here, a friendship, whose parties are jointly pressing towards a more truthful, more workable, or otherwise better understanding of what that friendship is.  That means that operating only on the basis of rationalization-by-desires condemns one to life largely empty of the goods that really matter – for instance, friendships.  To be sure, whether this is the cost of surrendering dialectical activities as Brewer describes them will depend on whether he is substantively characterizing those central human goods correctly; an opponent will presumably contest the view on those grounds.