One reason for not doing it is that a belt can cause an unsightly bulge under the waistcoat. Another is that a belt should be unnecessary because a kilt is a fitted garment with its own straps to hold it up.

Depending on the cut of the waistcoat, a belt buckle can look good peaking out from between the peaked bottom. The problem here is that it quickly looks sloppy and overdone if the belt twists or hangs too low.