One of the reasons that the "cannoli" line is in the film may be because the woman who played Clemenza's wife was Richard Castellano's real-life girlfriend. No doubt he pressured FFC and Puzo into giving her a line of dialog ("Don't forget the cannoli"), hence Clemenza telling Rocco to take the cannoli.
Allegedly, one of the reasons that Castellano didn't appear in GFII was that FFC and Puzo refused to write in a part for his girlfriend.