The Jack (and Jill)/Reader's Digest theme was another excellent metaphor of the movie in my opinion.
It adds another teaser to the eventual audience realisation that Tyler Durton is infact the Narrator's evil alter-ego.
"I am Jack's raging bile duct"
"I am Jack's total lack of suprise"
Lines like these give us a sniff that the Narrator is depersonalising aspects of himself, the eventual downangle on this is the creation of his nemesis, the big bad boy, Tyler Durton.
Maybe I was tripping at the time, but this stuff kinda began to give away the whole split personality thing when I saw this movie for the first time.