system
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human
CMV: Because Aragorn was able to summon an (invincible) army of the dead, Rohan's aid wasn't necessary for the victory at Minas Tirith

Pretty much what the title says. It seems like Rohan lost a lot of good men, including Theoden, when they rode in to save Minas Tirith. However, I don't really see that their aid was necessary in the end, because the dead army would have just saved the city anyways. I understand that Rohan's involvement augmented the dramatic effect and it was pretty epic to watch them almost completely wipe out Sauron's army. But, in the end, I don't think it would have even mattered if they were involved or not and, quite honestly, it seems like a lot of lives were wasted unnecessarily. Regardless of how dramatically epic the scene was, Rohan wasn't needed to save Minas Tirith; Aragorn was able to do it himself.
gpt
The army of the dead was changed both in role and in nature in the film adaptations.

In the original novels, the army does not fight in Minas Tirith at all; rather, they help Aragorn fight off the Corsais, servants of Sauron who owned a fleet of warships. Aragorn then seizes the fleet, releases the dead from their oath and sails to Minas Tirith with a very human army.

It is also heavily implied that the dead either cannot or do not inflict physical harm; they win the battle by terrifying the enemy into fleeing. Note that the corsairs were just human and it is doubtful that this would work against, say, trolls, uruk-hai and especially the Nazgul.

So in the novels, at least, they are nowhere near the weapons of mass destruction that they are in the movies, and Rohan was vastly more important.