Besides the above well-made points, a trouser belt is too narrow and will migrate to the top of the kilt. (Where it should not be). As you know, a kilt's waistline is located two inches down from the top, so a belt that is wider than two inches stays put. A trouser belt is often 1.5 or 1.75 inches. I suppose it would stay if cinched tight enough, but there would be a wide expanse of kilt showing above it. In any case, the belt is merely decorative.