Currently, it’s chance to provide you a free Malefic Rapture (no Soul Shard Cost and no Cast Time) is really only half true. Sure not using the Soul Shard is great, but the Global Cooldown after casting the ‘free’ one is the same (sometimes longer) than the actual Cast Time of Malefic Rapture. We’re basically waiting the same amount of time after an instant cast than we would if we had just used the cast time.
What I’d love is Tormented Crescendo instead stores charges (either 2 as it is now, or preferably up to 4), and your next Malefic Rapture cast expends all charges to either empower the damage of one Malefic Rapture, or better yet cast additional Malefic Raptures for every stored charge. It could improve cooldown windows, by not having to rush to pop a free TC MR which is soon expiring, you can hold for when you really want to use them.
For PvP it would mean you could be interrupted (sorry friends). But if the charges don’t expire until it is actually cast, and with the bonus damage being frontloaded, surely this would make up for it?