Why was the army of the dead at Minas Tirith?

[Tuor in Gondolin]

It's hard to say which was worse, the green slime at the Battle of
Minas Tirith or Gandalf assaulting the Stewart of Gondor in
direct contravention to his instructions. Actually, I suppose the
green slime, especially since in the movie there is no reason Aragorn
then lets them go, not even letting them get in a little dwarf
tossing. All it would have taken was a brief scene of Aragorn letting the green
slime go (even quoting the book), and rallying the Gondorians.
Btw, PJ and friends missed one of the great scenes in the book of
Eomer seeing the black ships Arwen banner and tossing up his sword
and the bells of Gondor ringing. (An echo of Churchill's having the
bells in British churches ringing [after Alamein?] )?