1) We got the hint (tail in prologue, eye in epilogue of AUJ). It's time for the full reveal.
2) We get to see Smaug's demise again when TaBA starts up. Think Balrog in TTT.
3) The main villain cannot be defeated before the secondary villain. (Yes, the necromancer is the main villain in TH, as Gandalf explains on the journey home from the Lonely Mountain). If the climax is the defeat of the necromancer in DoS, TaBA is just the mop-up crew.
4) Smaug dies at exactly 2/3 (chapter 13 of 19). This could fall either way, but defeat of the necromancer has to take place after Smaug's death; that will likely take a pretty good chunk of film time; and there is a single sentence (or maybe 2) devoted to his defeat in the later chapters. I think it is a good idea to move chapter 12 after chapter 13 (and into TaBA), anyway. PJ is not above moving events around for a better story.
5) It is more PJ's style to show you where the story is going instead of hanging off a cliff. We will see the humans discussing marching on Erebor, the Elves moving toward the ruins of Laketown, and the orc army on the march at the end of DoS.
6) There is no climax or resolution that centers on Bilbo (or Thorin) if Smaug is shown winging off to Laketown at the end of DoS. Killing Smaug points everyone in the direction of Bilbo and company. Bilbo can say something like, "Now, I guess the worst is behind us." Cue to the necromancer giving orders to march on Erebor.
7) Thorin's dragon-sickness arc kicks in at the start of TaBA. No reason to dilute it with a pointless battle with a secondary villain. The plight of the humans and the marching of the armies is the proper backdrop for Thorin's descent.
8) The death of Smaug enables Bilbo and company to see the destruction of Laketown. (PJ could very well have them viewing the battle from a distance.) We can get hints of concern from Bilbo as he notices Thorin acting in some manner that does not seem appropriate, and as the armies march.

P.S. I do not know what PJ is going to do, but either way, I think there will be common sense reasons to choose killing off Smaug at the end of DoS or the start of TaBA, depending upon the focus of the story. Trying to say that only one of the options is the "common sense" one smacks of arrogance.

Some one has made the point in this thread that PJ and Sylvester McCoy have basically confirmed that the battle of Dol Guldur is in film two.

If this is the case do the 'smaug-slayers' amongst you still think it would be possible to kill him off in film two as well?