I may be wrong Harold but it is my understanding that the leather flaps are originally made to cover the laces from mud or debris (of course purely stylistic on slip-ons). As they were/are typically found on outdoor daywear shoes, to me, they look out of place on evening wear.
I think you're correct MacMillan's son, but there is also a style of buckle designed to be fitted onto ordinary lace-up brogues for evening wear. I have a pair of these buckles but have never worn them. Here is one fitted. The external tongue covers the laces and the overall effect is very similar to Harold's brogues.

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