Ultimately, I think PJ did this simply for time. At that point we've had, what, 3 major battle sequences (Moria, Helms Deep, and Pelennor) and a handful of skirminshes, with another one to come shortly at the black gate. He needed a way to end the battle decisively and quickly so in a movie goers mind they would say "ok, that chapter is done".
IIRC, the dead don't even KILL anyone in the book. They literally scare the Corsairs away.
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