Never, those are a good frag!
That's what I thought earlier on but now I keep them around. I don't use them all the time but definitely useful. I primarily use it how
@Gladiola described: when a player expects to be away from the game for more than ten hours. Most of the time, I have been lucky to have a schedule that allows me to pop on even if it is just long enough to clear the bar. Sometimes, however, I know my schedule won't allow it and so this allows me to not lose any (loss in the sense of not getting what I otherwise would have gotten if the bar wasn't filled and waiting to be emptied before more could generate). Usually I am not too bothered if it sits full for a few hours, but I use it most often when I am new in a chapter and have lots of research that I want to activate as quickly as possible (but not bad enough to purchase the kp, which is a different controversy of sorts)--which is where I am at the moment.
Before every FA, I remind my FS to spell their bar ahead of time so that when the event begins two badges are ready for the making (I usually calculate in my schedule when to stop using the kp bar and let it fill for that timeline, none wasted with a full two bars either). It seems like this badge is one of the ones I have the most of leftover, have more than I have opportunity to place, so getting that one extra at the beginning probably isn't important but it feels good to hit the ground running (just like everyone who sets their longer factories in advance, badge ready when event starts). FA runs for about a week so I usually use three of them so that no kp is wasted when ready to make a wonder badge. The badge does not require all ten to arrive at the same time, just needs ten, but using my kp in tens for that is easier than tracking how many more that badge-in-progress needs. IM lets me take the kp out at full ten at a time, but zero wasted during clearing delays (if my bar is at thirteen, I just clear the ten and let the pile continue until it meets/exceeds ten).
It is a weak replacement for AW instants, but for a city that struggles keeping those instants coming in, and it is easier to craft these, then it has its value for them. (though I would still suggest CCs be the enchantment crafting priority, followed by MM, in my opinion)