I will put myself in the minority here about the fly plaid, however they are of limited use. Because of when I first got my formal wear, and how my family was involved with my local St. Andrew's society, there was a lot of information and a lot of misinformation. I regret not getting my fly plaid when I got my kilt. Since I bought them separately they are from different kiltmakers, different mills and it shows up close.

However I really only wore it once a year, there just were not the opportunity that I thought there would be. To my mind it ads a sort of intermediate level between black tie and white tie and does not fit into every event.

So I would say, if you can buy it now, and you think you will have some use for it, do so. Think about where and how often you will be kilted in a formal setting and decide accordingly. I can't say that this will ever be an always or never answer.