I personally didn't mind seeing all that stuff on Fontane in the novel, in fact, I never really cared much for the character in the film but I found him a helluva lot more interesting in the novel.

But yeah, I can see why some would deem all that stuff with Johnny, Nino, Jules, Lucy etc. unnecessary, but I guess Puzo wanted to show us how Vito's world had it's effects on even these seemingly insignificant characters.