It can be just a throwaway reference, showing him for a short time (like Sauron's armoured form).
Besides that though, here's one thing that definitely bothers me: the inconsistency in Smaug's portrayal.
We've got the four-legs Smaug...
...the Bat Smaug...
...and final Smaug.
Boy, I hope they just make one damn coherent version for the extended trilogy next year.