There is a point about that energy being useless in so short windows where swapping to 2x slow would be useful. It's great to expand kit with your rogue. General idea was that when player can recognize situation where that is useful - he probably already can do it. Mindlessly spamming FoK with 2x slow weps on Embers is just dumb, either player doesn't understand underlying porpouse or has a real hardon for recount numbers. But there are situations where it is useful, at least on paper. It's not a waste to learn it, hardest part is to see when it's useful.