[Elladan and Elrohir]

I think releasing the Dead is the honorable thing to do; maybe not the most strategic, but the most honorable. In the book, it never specifically says that they agreed to fight for Isildur in only one battle, but it does seem to me to be implied that that was their commitment.

I understand your point, alatar, that the Dead aren't as powerful in the book and thus they wouldn't be as much of a guarantee of success, but does this really make a difference? Doubtless they would have at the least helped at Pelennor Fields, or at Morannon.

Of course, I think one of the reasons Aragorn dismissed the Dead at Pelargir was that he couldn't transport both them and the living up Anduin in the ships of the Corsairs; for the Dead breed terror among the good guys as much as the bad. And even though their terror isn't emphasized in the film, isn't this a valid point that might also make it disadvantageous to keep them around?

Ultimately, as the King of the Dead points out, "You gave us your word." And he did. It was merely to "fight for us," but they fulfilled their oath nonetheless and deserved to be set free.