Gray's love for Iced Shell
Even if this joke will never not be funny to me, I have to concede that it was always a justified use. Except against Lyon (lmao wtf, purely emotional but dumbest move ever).
Deliora ? With all we knew at that time it was legit. Deliora was a menace, Ul had the level of a wizard saint and couldn't do sh*t against it. It was a miracle her spell weakened Deliora during those 10 years. Seeing this monster, who destroyed his village killed his parents and his teacher, being set free and roaming the earth once more, plus having all his best friends on the island stuck with it ; was a fairly understandable situation to crashout for Gray
Silver ? Once again, Deliora, plus Gray wasn't really gonna use it, it was a bait.
Zeref. Well, if this spell was meant to be useful one day I honestlty can't see against whom it would be better than against Zelef ( maybe Acnologia ? ). Zelef himself was a bit afraid because it was basically like a checkmate by sacrificing one of your rooks but without even knowing you had one to begin with (thanks to the Lost attribute)