Why Saruman Must Die at Isenguard - As much as we might hate it, it makes sense. Follow the logic.
This is a good example of why "clean lifts" aren't as easy a solution as we would hope, because even clean lifts leave ragged edges that must be cleaned up and "cauterized".
OK, here we go. Let's try to follow this from the scriptwriter's perspective:
1. 12 hours of movie cut to 6 hours requires 50% cuts. This is a given…the movie is approximately 12 hours is shot "as is" with all scenes intact from book. This we know from many attempts in the past (BBC production among them). Just for now, let's assume this is true.
2. To preserve the "important" scenes we must ruthlessly cut the less-important scenes. Friends, 50% means 50%. This is simple physics. We must cut HALF of the book to make the movie, and we must cut minor themes, unless we are willing to lose major themes, like Rohan or Moria, for example. Of course we don't want to lose major themes. So we must ruthlessly cut the less-essential sections.
3. Saruman in Scouring is non-essential. Again, we have to cut somewhere. Saruman in the Scouring is nice to have, but not absolutely necessary. We lose a theme, showing Frodo's mercy, but that's not the first loss we'll see in this movie. (In fact, some people have pointed out that Saruman in the scouring doesn't make much sense even in a book., but we aren't debating book-writing here, and we all like the book the way it is, so let's not get into that). In any case, the scouring is shrunken substantially, saving us a few precious minutes of screen time.
4. Saruman still must die. If Saruman is cut from Scouring, then what do we do with him? We still have to kill him somewhere, at least, if we want to stick closely to the events in the book. Leave him holed up in Isenguard with the Ents guarding him forever? Makes no sense at all, and a good example of a deviation from the book which does NOT work. See what I mean? Clean-lifting Saruman from the Scouring leaves a ragged hole. How do we clean it up?
5. The obvious place for Saruman to die is Isenguard. If he isn't going to the Shire then there is no need whatsoever for him to go into the 3rd movie at all. It would be a waste. So, we have to kill him off at the End of the Isenguard scene.
6. How do we kill him? Well, everybody hates the spike thing. But, what's the alternative? Clearly he has to fall out of the tower somehow. It makes no sense for him to come DOWN when we know he hates the Ents. Sure, we could just have him drink poison, but that is completely inconsistent with his character.
7. The spike is as good a plot device as any. He dies on his own machines. Maybe he is pushed by Worm (who also must be killed then, later on.) This is my guess, because it is the "closest to the book". Worm hates him, Worm is the vehicle of his doom. This part of the book is preserved.
8. Yeah, it's a little gory. But, isn't "having your throat cut," as in the book, also very gory? You can debate this all you want, but still, I don't see a lot of other good ways to kill him off. So what are we left with? The scriptwriters had to find a solution. Some of us may hate it. But there is no easy "out."
Again, I can't prove any of this. Again, I personally don't like too much gore, and will be turned off if this is representative of the whole movie.
But the bottom line is, this scene is logical and in some ways unavoidable. The cuts have to happen somewhere, and there must be lots of cuts. Even "clean lift" cuts leave holes in the plot that must be stitched up with "artificicial" plot devices. We want those plot devices to be a close as possible to the themes of the book, and also be exciting and movie-like. That is the challenge of adapting a book to movie.
And most of all, we all need to understand that this is just the beginning. A 50% cut ratio requires many themes to be cut. This is not trivial.
So what is next: The Palantir? Paths of the Dead? Gimli's speech in Elrond's Council? The Pyre of Denethor? Frodo's ordeal in the Castle? The Grey Havens?
What else will get cut? Whatever it is, I'm sure it will provoke outrage by fans who want their favorite scenes left in. Unfortunately, outrage won't change the laws of movie physics.