I know it was already asked, "Why didn't Aragorn keep going with the army of the dead?" and the obvious answer that he gave his word and had to keep to it. I'm not sure why somebody didn't read what I wrote here and forced me to edit it again. I had already read that question and all of the answers. It is substantially different. I know that Aragorn made a promise. I know they were traitors. I know they had to aide the king of Gondor to get set free. I know Aragorn had to keep his word...however I completely disagree with the comment below that it wasn't Aragorn's deal to make. In fact it was Aragorn's deal to make, since he made it. How would they know in the future exactly what was needed from them and when? They did not know that a king would arise to offer them a deal. In fact they thought Isildur's line had ended. So it was a surprise when Aragorn went to them asking them for aid. So again since it was Aragorn's deal to make...
Why didn't Aragorn simply tell them they had to fight for him until Sauron's armies were defeated? Or tell them to skip the battle at Minas Tirith and go straight to the black gate and take out Sauron's main army behind the black gates?
We're talking about Aragorn, a very wise man who was over 80 years old at the time. He knew already how difficult Saruman's much smaller army, without the aide of the Nazgûl, was to defeat in an area that was smaller and easier to defend. Why simply ask for aid in a single battle?
Also related to this is that one the way to Minas Tirith they did aide him additionally on the ships. So was his releasing their oath really only tied to this one battle, couldn't he have sent them on as a continuation of the same war that needed to be concluded?
Why didn't Aragorn cut a better deal? And was the deal really that limited?