It IS the heart and soul of the film, absolutely. Ennis holding Jack in their favourite place, Ennis is so happy there with Jack. It says so much without words.....so beautifully filmed, it absolutely floors me every time I see that scene. I believe that this scene encapsulates everything about how they felt about each other, but mainly how Ennis felt about Jack, because Ennis found it so hard to express how he felt, so to see him, holding Jack, speaking softly to him (I adore his voice here. It is so beautiful, low, soft, tender and loving), it says everything and we KNOW he loved Jack in the same way Jack loved him. It also goes some way to explain why Ennis reacted the way he did when they had to leave Brokeback early (using the excuse of the money), and why he breaks down in the alley way. He never wants to leave Brokeback, because by leaving Brokeback, he's leaving Jack, the love of his life.
To cut this scene into the final scene of them together, illustrates so clearly that Jack loved Ennis then, and he will love him now, whatever happens. He will take from Ennis anything that he gives him, no matter how small. This is how I see the whole scene making sense.