A View of Denethor's Death
Peter Jackson's version of Denethor's death has, like every moment of the movies, been picked apart and analyzed in minute detail. The view of Denethor running and jumping from the tower of Gondor in flames has prompted criticism and laughter among the LOTR Internet community. The general consensus is that Jackson went overboard on this moment and fell into the trap of spectacle over substance. That was my first thought too.
A second viewing of the film brings a different perspective, however. Notice the way the camera pans away as Denethor's body falls. The fireball grows smaller and smaller until it is a mere speck of light. At the same time the citadel of Gondor fills the screen, beginning with the high court and growing until all seven levels are revealed along with the field of the Pelennor covered with the hosts of Mordor.
It is at that instant when Peter Jackson's point becomes clear. Denethor's death is only one more among hundreds of deaths in the battle. Because his mind has become so deranged Denethor sees his own death magnified in importance, hence his determination to end it in ceremony, "burn[ing] as the heathen kings of old." In reality, his death is meaningless and unnecessary.
On the other hand, Denethor's death is significant in several ways. In committing suicide, he leaves Gondor without a steward, since Faramir is too ill to command the citadel. He shirks his duty, which Gandalf actually spells out in the book: "it is your task to keep some kingdom still against that event [the return of the king], which few now look to see" (ROTK Ch 1). This does not begin with his death, though. By overreaching his power and authority, Denethor earlier fell into Sauron's grasp, losing his mind and becoming a pawn in Sauron's battle plans against Gondor. Had Denethor not grown in pride, and therefore blindness, perhaps some of the grief of the siege could have been averted (Faramir's tragic attempt to re-take Osgiliath comes to mind as the main example).
Denethor's death affects the Battle of the Pelennor in other ways that can only be seen in the book. When Pippin asks Gandalf to try to save Faramir, Gandalf replies, "Maybe I can, but if I do, then others will die, I fear" (ROTK Ch. 7). This prophecy comes true as Theoden dies at the hands of the Witch King while Gandalf is in the tombs with Denethor.
In a movie, battle scenes cannot be slowed down with the narration and dialogue that clarify Tolkien's story. Movies rely on visual imagery instead. And Peter Jackson is the master of that technique. By showing Denethor as a diminishing plummeting fireball against an epic battle for a background, he conveys with perfect clarity the folly, the insignificance, and the pitiful tragedy of his suicide.