One of the interesting things about "weak" Fredo is that he's a throwaway character in the novel and in GF--but not in II. In both the novel and GF, he basically has two scenes: his breakdown after Vito's shot, and his taking sides with Moe Green in Nevada. IMO, Puzo's purpose for Fredo was to a) flesh out the Corleone family; b) provide a raison d'etre for Michael stepping in to whack Sollozzo and McCluskey and then to assume leadership of the family after Sonny's murder; and, c) to enable Michael to give that "never take sides against the family" speech in Nevada, underscoring that Michael had made the transition from war hero who wanted nothing to do with the family, to Don-in-waiting, totally embracing Sicilian tradition.
And, though neither Puzo nor FCC could have known it then, Fredo's marginality as a character provided a powerful jolt when he turned out to be the family traitor in II. Turnabout's fair play...