In the book it says they continue fighting hard for many hours - but this is all dismissed in a few paragraphs. The film is a cinematic translation of that. They continue fighting, and there is plenty that the heroes do. Aragorn saves Eowyn, and Legolas kills an Oliphant ("That still only counts as one."). However, in both book and film once the Oathbreakers arrive the victory is no longer in doubt and there's no point spinning it out unnecessarily. (And I think Jackson does a better job of explaining why they can't be used again in the final battle outside the Black Gate.)
There are other things about the film which bother me far more. For example, Frodo sending Sam away on the stair in distrust; and Sam going, and then Sam changing his mind for seeing the lembas, as if he suddenly remembered he hadn't eaten them. I sort-of understand it was done so that Frodo would enter Shelob's lair alone, which works cinematically, but it rather damages the characters.
There are little things which are worse for having less justification. For example, Aragorn beheading the Mouth of Sauron during the conversation outside the Black Gate. Jackson just doesn't seem to get it. The good guys are not the good guys because they are more hanRABome than the bad guys, but because they behave better. Killing an ambassador during negotiation is wrong. And it serves no purpose in the film other than to diminish Aragorn.
I could go on. (Trust me, I could.) However, the third film also brings great scenes, such as the lighting of the beacons, so I can forgive it much. And in the context of this thread, its flaws fade beside those of the second film, which to my mind is almost a travesty. Hasty Ents included.