Again, from a cinematic point of view, Tom Bombadil making an appearance in the film would take too much time. Let’s face it: he really didn’t push the plot forward in the book either (though thematically, he’s an important character). Additionally, his appearance in the book, for me, drastically changed the mood; he’s a character that came out of nowhere, and seeing him in costume would’ve killed the film.
So yeah, Tom Bombadil just wouldn’t work cinematically. I believe that if they included him in the film, we’d have another Deathly Hallows Part 1 in our hands; a film which includes a whole lot of “character development” at the point of stunting the story’s pacing.