The Army of the Dead wiping out Saurons army on the Pelennor Fields was one of Jackson's worst decisions.

In the books, Aragorn enlists the help of the dead, and they basically scare the dudes off of the Corsairs and the men of the west take them over.

This bullshit "You and who's army??" "This army" (har har) ridiculousness, then the army of the dead just wiping out the whole of Sauron's army basically diminishes Rohan's role to nothing. Rohan could have stayed at home and nothing changes. The undead decimates Sauron's army either way.

PJ made some changes that actually work well in the movies, but damn this one was a huge miss.