[The Might]

All the important points have already been mentioned, I especially like Rumil noticing that without Smaug's death the goblins would have been much stronger. Perhaps any passage through the Misty Mountains would then be impossible.

Just wanted to add that I do expect Smaug to ally with Sauron. In LotR Sauron sends a Nazgul to Erebor to force the Dwarves into submission, but without Smaug's death he may have sent the Nazgul to arrange a treaty, perhaps assuring Smaug that he will receive his fair share of any encountered treasures. Since Sauron wasn't interested in gold and jewels anyway, he would have no problem with this. And Smaug would surely gladly accept.