Actually, in the book, when Sam looks in the mirror, he sees what could/may happen to the Shire. The movie basically has Frodo seeing what Sam had seen. I would rather have had Sam do this to add some motivation for his following Frodo to the ends of the earth, which is why it was like that in the book, in my opinion.
They easily could have included Sam in this scene without adding too much time to the movie, I thought. Just as they could have easily added the giving of gifts at Loth-Lorien without adding to much time, as well.