Well, Smaug isn't wearing any pants. ;)
Seriously though, you never know with the MPAA. (It takes just one F bomb to push a PG into a PG-13.) It could all boil down to how many whacks it takes to remove an Orc's head or something.
Revenge of the Sith is the only PG-13 Star Wars film to date, and that's mostly because of Anakin's BBQ.
Considering the graphic violence one can see on tv these days, an R rating seems kind of silly for BOTFA.