One of my favourite scenes in RotK is when the beacons are lit, and Aragorn comes sprinting in, yelling "Gondor calls for aid!" ... "And Rohan will answer!" and you see them all preparing to head off for war as Eomer rouses them up, while it clearly weighs heavy on Theoden's mind as he knows what lies ahead. And good god, the ride of the Rohirrim itself, that never fails to stir my heart and set it pounding. How utterly fantastic it was.

...then the deus ex machina ghosts show up, and the sacrifice of the Rohirrim, their charge headlong into the world's ending was worth precisely nothing and made no difference whatsoever.