the intent important to understand how the quality of their story was compared to their intent
But I think that only matters if you care about how well they did what they meant to do. If the question is, instead, how good a horror novel it is, does it matter if the author set out to write about romance? (This is assuming you just get a novel, and then, say, later you find out that most people think it was written to be a romance.) I've spent hours and hours of class time talking about whether Virgil meant to undermine the Roman Empire--but I don't think that matters; what matters is what the Aeneid has to say about empires.
Let us say, for instance, that Lewis had included a terribly unchristian scene (er, imagine what you will). Would his intent to write the Chronicles as Christian literature make that scene any less problematic? (And, of course, whether or not Narnia is Satanic is all in your definition of Satanic, which is something that none of Lewis's intentions will ever be able to fix.)