Remember, I said Tolkiens
sam works well in the book but not on screen and the same is true for pj's sam.
Tolkiens sam makes mistakes that endanger the "Mission, quest, thing" (I love that bit), while
frodo does not. Even with the rings effect on him, he is able to hold his tounge while sam acts like a giddy schoolgirl on her first date. The same is true at the meeting with
faramir. Frodo is in control, while sam puts his foot in his mouth. It is no wonder that Faramir put his pressure on sam when sam spilt the beans about
the ring. He knew that sam was a motor mouth and frodo was not. This is how
tolkien wrote sam.
Now, for PJ to add that part of sam into the movie, the viewer would not view sam as the trusty companion, but as a bumbling boob that could not be trusted. I remember in the book when sam ended up with the ring wondering how this idiot was going to complete the "thing". Then, when he finds Frodo again, it was totally un-believable to me that he actually succeeded in rescuing him.
Viewers of any visual medium don't want to go on a character roller coaster ride. Sam is smart, sam is not, hows he gonna pull this off, How did he pull that off. They spend their time focused way too much on sam and the finer points of the film are lost. Look at the movie "Casa Blanca". Rick (HB), was the same tough selfish character through the entire movie, but in the end, becomes what he was in the early days. simple and straight forward. and how many characters are really developed? 3. The rest of the actors are one way at all times.
So to put this much developement into sam would take away from
aragorn, frodo,
gollum (probably the most key character to develope), et al.
Sam, as a character, was basically a loyal friend. This has to be pj's focus. PJ's sam has accomplished that.
Tolkiens would not have.