Aug 31 2013, 1:48pm
(which is the basis of what is been discussed, I think) would be simply because he wants to cut the number of times Bilbo goes up and down the tunnel to Smaug's lair. The BBC radio adaptation did this by cutting out the theft of the cup altogether, and having Smaug be aware of Bilbo the first time he comes into his presence. So, I could see Bilbo finding the Arkenstone on his first visit, which awakens the dragon, and the riddles with Smaug conversation takes place there and then...
One reason for PJ to replace the cup with the Arkenstone...
But if PJ is keeping the sequence of events as written, then replacing the cup with the Arkenstone would cause problems because with the cup, Bilbo showed his prize to the Dwarves, and earned their praise for his courage and daring before bein sent down the tunnel again. He's hardly going to show them he's got the Arkenstone. In that sense it is better for the Arkenstone to be found by Bilbo exactly as in the book - after Smaug has flown off to Laketown and the Dwarves are all exploring the hoard.
"Choosing Trust over Doubt gets me burned once in a while, but I'd rather be singed than hardened."
¯ Victoria Monfort
(This post was edited by Eleniel on Aug 31 2013, 1:50pm)