Mainly on the Cub, the pull-pull is for scale looks, but it's used usually to allow the servo to pull in both directions and avoid control rod flex when the servo has to "push" against the resistance of airflow against the control surface. I agree with using the metal control rod for the elevator; on a Cub there shouldn't be any problem with flex, but at least the metal rod won't expand or contract and throw the trim off. I have to adjust my Sig Cub elevator trim almost every time I fly it.