On the contrary, I believe it is the central message in the book. The Road is about a father's love for his son and about the overcoming power of love and goodness in the face of darkness. The first obvious mention of "carrying the fire" may not come until mid way through the book, but by that time the groundwork for its symbolism has been laid.
On the first page of the book we are told of a dream the father had in the night.
"In the dream from which he'd wakened he had wandered in a cave where the child led him by the hand. Their light playing over the wet flowstone walls."
From this we can't draw much. But at the end of the story the dying father is revisited by this dream and we are told that, "The light was a candle which the boy bore in a ringstick of beaten copper."
Why is this important?
Throughout the story the father struggles to know whether he should keep the boy alive or not. He lives in anguish watching his son suffer. He has no clear picture of why they are moving on and why they are fighting to survive. But at the beginning of the book McCarthy says, "He knew only that the child was his warrant. He said: If he is not the word of God God never spoke." The word "warrant" has a few definitions but the one implied here is, "an assurance, or guarantee, of some event or result. Justification, grounds for an act or belief." The child was his God- given assurance or guarantee of some result that would come from keeping his son alive. And that was all he had. And throughout the book he wrestles and struggles with it. And so throughout the story he tells his son that they are the good guys and that he is carrying the fire. And then at the end of his life he is given the vision to see that his efforts were not in vain and that his son was really, truly full of light.
"...and he would raise his weeping eyes and see him standing there in the road looking back at him from some unimaginable future, GLOWING in that waste like a tabernacle." (emphasis mine)
"He watched him through the grass and kneel with the cup of water he'd fetched. There was light all about him."
"He took the cup and moved away and when he moved the light moved with him."
"Is it real? The fire?
Yes it is.
Where is it? I don't know where it is.
Yes you do. It's inside you. It was always there. I can see it."
The father believed in some sense, I think, that the fire his son carried was real, but now he can see it. I think the vision given to him is a gift. He can die now knowing that the child was his, in fact, his warrant, he was the word of God, and God had indeed spoken.
So that brings us back to the dreams. Why is it important? Well, in the first dream we are told that the child is leading him by the hand. And then right before he dies he has the same dream in which we are told that the light they had in the dark was a candle that the boy carried. When combined, the father sees that it was actually the boy who had been leading him all along by the fire he was carrying.
Candle on a ringstick=carrying the fire
Yeah, so not baloney.