I think the matching cummerbund (or waistcoat) elongates the leg line, thereby giving the wearer a slimmer silhouette. I don't get why you guys want to ditch that benefit by wearing a non matching cummerbund or skipping it all together.
Traditional formal dress is already optimized so you look your best. It's a mystery to me why so many people want to reinvent the wheel and think they will do better. There are some people who can pull off a unique look, but most can't.