It will make the Thorin's decline and his treatment of Bilbo MORE impactful. We can see how much he trusts Bilbo now, and when Bilbo gives the Arkenstone to the "enemy" (in his mind), he will feel even more betrayed. I think this scene added a depth to his character that wasn't there before...he's not just a stoney, would-be king who only cares about reclaiming his kingdom, he has a "soft" side (for lack of a better word) that doesn't come out very often, but it's there. I think this will make his downfall even more heartbreaking, because even though he's stubborn, he's not completely unreasonable...and he will become so with the dragon sickness.