The cummerbund heightens the waist of the tuxedo trousers especially when worn with an eton style jacket without a waistcoat. This prevents an expance of shirt being seen between the top of the trouser and the bottom of the jacket. A traditional kilt is worn where the top of a cummerbund rises, hence there is no need for a cummerbund while wearing a kilt. Like P1M says, the cummerbund would be up around your armpits if worn kilted.