Ultimately, it's not that I think fiction can't shape people's perceptions and actions, it's that there's never room for the concept of critical engagement in the discussion, at least in the sense that I mean it. When tumblr people say "critical engagement" they typically mean something at most a couple steps short of "book-burning," but it's obvious to me that it is possible to enjoy a work of fiction while repudiating the values that inform it, and that in fact engaging with works can allow for more effective critique of the values expressed within them. I'm much more able to argue against, say, G.K. Chesterton's theory of history and blind glorification of war now that I've actually read him than I would be if I just knew in some very general sense that he was a Christian conservative.
Moreover, the literary fuck/marry/kill game that the benighted denizens of this website like to play obscures the importance of engaging critically even with texts and value sets that are generally in the right. Very few people are right about everything, and even aside from that it's worth recognizing bad arguments for good ideas.