That action sequence was to satiate the desire to see the dragon do something before the end of the movie. I just watched The Desolation recently and I have to say that even though I went into the movie with low expectations I was still disappointed.
The amount of pointless filler in these movies is approaching legendary levels. I mean, with the length of all three films combined they could easily have done a page by page rendition of the hobbit. Hell, by the time the third movie comes out, I could sit down and read the hobbit quicker than watch it.
But length isn't the problem, the problem lies in the incredibly anemic character development and lack luster pacing. I was really bored with Smaug's scooby doo chase through Eador. (Actually if they had played the scooby doo theme during that whole sequence I would have found it much more enjoyable.)
The entire point of The Hobbit novel, the central theme, was that even meek individuals, who might not seem like much at first glance, can sometimes summon up incredible courage and wits and prove to be more reliable than the most heroic of champions. Yet in each scenario where Bilbo displayed his wit and cleverness in the novel, these movies choose to devolve those scenes into unnecessary action and chaos in order to provide some sort of visual thrill. It's cheap and over done.
So yeah, I am not looking forward to the last installment. I really can't see any way to redeem these movies at this point. No matter how many cpu's they sacrifice to the CGI gods in order to make the battle of five armies sequence last an hour and a half, I have little interest to see it.