I like this analogy. I think that George Bernard Shaw said that all good literature is didactic; @Christy or @Laura could correct me here. That fits into the purpose of sermons well, too, I think.
C S Lewis liked it, too, as the following quotes remind me:
From “The Voyage of the Dawn Treader”:
"On the next page she came to a spell ‘for the refreshment of the spirit’. The pictures were fewer here but very beautiful. And what Lucy found herself reading was more like a story than a spell. It went on for three pages and before she had read to the bottom of the page she had forgotten that she was reading at all. She was living in the story as if it were real, and all the pictures were real too.
When she had got to the third page and come to the end, she said, “That is the loveliest story I’ve ever read or ever shall read in my whole life. Oh, I wish I could have gone on reading it for ten years. At least I’ll read it over again.”
But here part of the magic of the Book came into play. You couldn’t turn back. The right-hand pages, the ones ahead, could be turned; the left-hand pages could not.
“Oh, what a shame!” said Lucy. “I did so want to read it again. Well, at least I must remember it. Let’s see . . . it was about . . . about . . . oh dear, it’s all fading away again.
“And even this last page is going blank. This is a very queer book. How can I have forgotten? It was about a cup and a sword and a tree and a green hill, I know that much. But I can’t remember and what shall I do?”
And she never could remember; and ever since that day what Lucy means by a good story is a story which reminds her of the forgotten story in the Magician’s Book.
“Oh, Aslan,’ said Lucy. ‘Will you tell us how to get into your country from our world?’
‘I shall be telling you all the time,’ said Aslan. 'But I will not tell you how long or short the way will be; only that it lies across a river. But do not fear that, for I am the great Bridge Builder.”
From “The Horse and His Boy”:
“Then it was you who wounded Aravis?”
“It was I”
“But what for?”
“Child,” said the Voice, “I am telling you your story, not hers. I tell no one any story but his own.”
From “The Last Battle”:
“And as He spoke, He no longer looked to them like a lion; but the things that began to happen after that were so great and beautiful that I cannot write them. And for us this the end of all the stories, and we can most truly say that they all lived happily ever after. But for them it was only the beginning of the real story. All their life in this world and all their adventures in Narnia had only been the cover and the title page: now at last they were beginning Chapter One of the Great Story which no one on earth has read: which goes on for ever: in which every chapter is better than the one before.”