: Discuss.

I like the question you have raised - interesting.

I would say that both Saruman and Denethor were less "Enslaved" by the influence of Sauron, through their use of the Palantirs, than "Tempted". Both characters thought that they were acting in their own interests, rather than Sauron's.

In some ways Saruman and Denethor can be seen as opposites. Saruman, who should have acted as a servant and mentor to the cause of the "Free Peoples", was tempted into evil acts by his desire for power.
Denethor, in contrast, a powerful figure - the leader of a large and still comparatively strong state, was tempted into acts of dispair precisely because he saw himself as a servant of the people and state of Gondor. He saw what he loved and cared for most in the world threatened by irresistibly overwhelming forces.

Denethor was affected by the Palantir in two main ways. First, he had direct and intimate knowledge of what military might was ranged against Gondor. Second, his contact with Sauron sapped his resolve to fight on against the odds he saw stacked against him. Sauron had a long history of using pride as a weapon aginst his opponents (Ar Pharazon in the downfall of Numenor), Denethor's overweening pride could not allow him to see the ruin of his land and people so he was induced into suicide.

The Palantirs were a powerful tool for Tolkien's plot development. Without them some possibly rather jarring incident, such as Saruman being captured by Sauron, would have had to be used to rationalise Saruman's defection to the forces of evil.
The existence of the Palantiri also allowed one of our closest encounters with Sauron via Pippin, the other being the "Mirror of Galadriel" episode.