I assume that the filmmakers wrote the dialogue that way for dramatic effect. My impression was that they had the characters say the words almost all the time, but that in most fight scenes they skipped out on most of the incantations in order to focus more audience attention on the action. For example, in the book version of the final fight between Harry and Voldemort, Voldemort uses the Killing Curse while Harry casts a Disarming Charm. In the movie, the first detail can be presumed while the second one makes no sense in context (in the book, Harry gave Voldemort a last shot at redemption, and it was thought of as his signature dueling spell), so the scriptwriters dropped it.
The movies, as far as I can remember, completely passed over the idea of casting spells without saying the words.
EDIT: Yes, this was done in the fifth or sixth book and afterwards. But they never covered that in the movies, so for a movie-only audience the ability doesn't exist. There were also combats where Harry would say one or two spells and not the rest. This really stood out for me in the fight scene at the end of movie 5.