[Luthien_ Tinuviel]

I think it's an effective (though perhaps not conscious) narrative device that as the journey gets harder for Frodo, and as they move closer to Mordor, the story focuses more on Sam. It is already incredibly hard for Sam, but it must have been infinately more harrowing of a journey for Frodo. The chapters from about Shelob's Lair (actually a little before) on are very dark, and I always find the first three chapters of book six rather hard to read, because the journey is so dark. If it is emotionally draining for the reader to see the journey from Sam's perspective, Frodo's point of view would have been unbearable.