I
had thought that the Shelob episode worked quite well being moved to RotK. But reading what everyone's saying here is making me change my mind about that. It's true that the ending of TT was a little strange and subdued, and it would have made a wonderful ending to have left the non-reading audience thinking that Frodo was
dead.
There would have been another benefit to it of course, and it may have made even more people go out and read the books.