Besides being hard to understand, even for those who have read the book, Tom was probably also left out because he does
nothing else after the Hobbits leave the 'Downs. I can think of only two other mentionings of him: another place the Ring cannot go and a place for Gandalf to be instead of helping scour the Shire (which I understand the Hobbits had to do on their own).
This is similar to their reason for leaving out Glorfindel, but that's another rant.
If they were going to make Tom like the card, then I'm glad they left him out. That one's not quite my mental image, nor for Goldberry (needs longer hair, definately needs longer hair...). Anyway, because Tom's so....Tom-like, we all not only have our own images of him, but we also have our own understandings of him, which are sure to differ from the film-makers'; so even if they got our image right, the character probably wouldn't have been 'right'.
So here's to our mental images of Tom, Goldberry, Old Man Willow and our beloved Barrow-wights who will remain undisturbed:
