Think of a film like a song. Throwing the most awesome riff in the universe onto any old song won't necessarily enhance it, but in many cases throw it out of whack or just make it noise.
A classic case in point is Lord of the Rings. Copying and pasting the book from page to film, word for word, would've been a disaster. Hence many scenes were cut that weren't crucial to the plot. On top of this, each film had at least 30-40 minutes cut out for cinema release. Even with the special editions on dvd, there were still scenes that never saw the light of day. The mantra was clear, focus on Frodo and the ring, and only go to the others when absolutely necessary. This kept it lean, while still being epic.
A case of doing it wrong ironically was Peter Jackson's King Kong, where a good hour of the film had nothing to do with the story, and even though they may have been cool scenes by themselves, they pull you out of the story. Bay's TF sequels are another example, but in this case it was a case of too many drawn-out action sequences, "battle fatigue" kicks in and you lose interest.
Because geeks like us gobble up every moment with our favourite characters, we assume that having more will enhance our experience. The opposite is often true.