Do you not think though, that wearing a supernumerary belt with a kilt that is already supported by straps probably started as an attempt to get the look of an older kilt that was held up by a belt, and continues because people think it's the thing to do, without conscious thought?
Well, if you are asking whether the wide waste belt continues to be worn for fashion even though it really doesn't have a function, I'd agree with you and say yes. Keep in mind, though, that for a lot of us, the belts we wear with our trousers fit the same description. My pants stay on just fine without a belt, but I usually wear one anyway, for purposes of fashion, not function.

Since I wear my kilt belt for fashion, too, you'll find that I wear it sometimes and don't wear it at others, depending on what kind of look I'm going for that day. Since it's not needed to keep my kilt on, I'm free to wear it or not based on what I think is going to be the best look for that outfit.