The army of the dead would turn around on Aragorn and his remaining guys and consume them or something. The oath breaker king ( in book) is like "release us", but the context shows us he means "release us OR...." they're just barely under Aragorn's control. They're not a weapon to be put away and then used again later, not even against Sauron. They're poisoned and corrupt beyond redemption. All Aragorn can do, after they keep their oath to him, is give them permission to evaporate into the void. EDIT: I just flipped through ROTK and can't find a scene where Aragorn dismisses them. They never speak to him, he orders them verbally to follow him to the Stone of Erech. I mixed up the book and movie, it seems.

Yeah, I know that's how it's explained, but... why though? Why would fighting in one battle fulfill their oaths? Presumably, Isuldur wanted them to fight for the whole war, that's how conscription usually works. So I don't see why one battle would do it, especially since as undead they have nothing to lose.