[The Saucepan Man]

I believe that, in the book, the Dead were bound by their oath only to fight one battle and so Aragron was bound to discharge them after that battle, so the film followed the book in this regard save that the Dead's battle was at Pelennor rather than Pelargir.

And what difference does it make whether the battle is won by men led by Aragorn or the Dead Army brought by Aragorn by virtue of his line? Either way, it is Aragorn's arrival which saves the day. there is enough elsewhere in the film to show him as a leader of men.