That... That right there made me hate the entire trilogy.. So we're slowly overwhelmed, we're slowly losing and being killed and the situation is so dire and... Oh there is the immortal deux ex machina guys just trampling over the enemy killing them all in a minute aaaaand we're done!
It would be more like, they invented the atomic bomb, use it to take Iwo Jima and then the United States goes back to sulking and wondering how the hell they're going to take the home islands.
In lotr they use the undead to win the battle of pellenor fields but they don't use it to win the war. It's a legitimate plot hole.
To be fair the entire army itself was just a distraction so that frodo could destroy the ring. The ghosts weren't what caused them to win, they just prevented the death of the bait.
Right but, why bother with smoke and mirrors when you have an immortal unbeatable army in the first place.
Even in the book its not really explained why the oathbreakers only help out for one fight; the only difference is in the books they never actually go to Minas Tirith they just help against the corsairs.
Presumably Sauron could've destroyed the ghosts, he was just a little distracted at that time frame, or not yet able or willing to face them directly. Like the Eagles, they can't really tackle the Sauron problem, so aren't as much of a solve as they appear.
Also the reason they only fight in the final battle I think is because that is the only battle they're bound to fight in, I'm not even sure you could use them to fight any other battle than against Sauron's armies.