To say that Bilbo's breath was taken away is no description at all. There are no words left to express his staggerment, since Men changed the language that they learned of elves in the days when all the world was wonderful. Bilbo had heard tell and sing of dragon-hoards before, but the splendour, the lust, the glory of such treasure had never yet come home to him. His heart was filled and pierced with enchantment and with the desire of dwarves; and he gazed motionless, almost forgetting the frightful guardian, at the gold beyond price and count.

...At last he looked down upon it and he caught his breath. The great jewel shone before he feet of its own inner light, and yet, cut and fashioned by the dwarves, who had dug it from the heart of the mountain long ago, it took all light that fell upon it and-changes it into ten thousand sparks of white radiance shot with glints of the rainbow.
Suddenly Bilbo's arm went towards it drawn by it enchantment. His small hand would not close about it for it was a large and heavy gem; but he lifted it, shut his eyes, and put it in his deepest pocket.
"Now I am a burglar indeed!" thought he. "But I suppose I must tell the dwarves about it-some time. The did say I could pick and choose my own share; and I think I would choose this, if they took all the rest!" All the same he had an uncomfortable feeling that the picking and choosing had not really been meant to include this marvellous gem, and that trouble would yet come of it.

...But also he did not reckon with the power that gold has upon which a dragon has long brooded, nor with dwarvish hearts. Long hours in the past days Thorin had spent in the treasury, and the lust of it was heavy on him. Though he had hunted chiefly for the Arkenstone, yet he had an eye for many another wonderful thing that was lying there, about which were wound old memories of the labours and the sorrows of his race.

until the end of the book. He seems tacked on in order to give Thorin some reinforcements. I can accept the explanation that a frontal assault would stand little chance against Smaug, but why wouldn't someone at least mention the dwarves who live in the neighborhood? And why wouldn't there be any communication with such dwarves?

"We were on a journey to visit our relatives, our nephews and nieces, and first, second, and third cousins, and the other descendants of our grandfathers, who live on the East side of these truly hospitable mountains," said Thorin, not quite knowing what to say all at once in a moment, when obviously the exact truth would not do at all.

But it's interesting to me that the subject of Dain and his dwarves doesn't even arise during any of the planning at Bilbo's house, Rivendell, Beorn's house, or Laketown. It's as if they don't exist until Smaug is dead and the Lonely Mountain is under siege. Then, "Oh yeah, Dain!"