I think this is one of the cases where you see Jackson not thinking about the effects of changing something, and then being stuck in a tough spot and being forced to make another change.
I have read from a couple places (and a couple of the different actors/actresses) that Jackson did not have a clear plan. It seemed like a 'rollercoaster' where things got decided days, sometimes hours before, and he just kind of went with whatever floated his boat. I think the Army of the Dead is another example of poor planning.
Making changes isn't a great disaster, as long as it is thought out and you aren't changing something for the sake of change.
Jackson, for some reason, decided to greatly inflate the numbers of Sauron's army, to 250,000 - 300,000, while keeping Gondor's and Rohan's forces relatively the same. Before Theoden sets out, he says he has 6,000 spears, that is what he sets out in the books with, and Gondor's forces are downright slim. Aragorn tells Elrond in Dunharrow (for some other unknown reason) that there "are no more men."
The problem becomes how does some 10,000 men defeat 250,000 if there are no more men? Well I guess we need an unkillable wave of slime. This is probably my greatest frustration with the movies, is making changes for the sake of change. And sometimes, the changes that first seem small and insignificant, effect the movie later on and force even larger and more severe changes. That is the effect of poor planning.
