Well, not for me thanks.
I was once offered the loan of a fly plaid at short notice to wear to a dinner, which I declined as I was uncertain how to wear it properly. I think one of these sashes, or a fly plaid worn incorrectly or on the wrong shoulder, just looks silly, but a fly plaid which the wearer has taken the time to learn how to wear it correctly can look very smart.
I would wear a proper fly plaid on a formal occasion and indeed a fly plaid is on my current wants list, but I would not wear one of these sashes.