I will play Killing Wave if my deck needs a high-variance card to improve its chances of winning. I have played with this card before (I understand this is misleading, but I regularly do play cards I call unplayable) and seen others play it during testing, store drafts and on MTGO. This card is highly situational. Do I want this card in my opening hand? Most of the time, no. Does my deck get better if I could draft multiple Killing Waves, let’s say 2 or 3? Probably not.

Avacyn Restored Limited is a format in which board presence dictates almost every game. If you manage to continuously have an advantaged board, there aren’t many cards your opponent could even draw let alone play out, especially considering the amount of cheap offensive tricks and tempo plays available.

I want to differentiate two cases here: In the first case, you have Killing Wave in your hand for multiple turns and have a chance to engineer its effectiveness and maybe, success. Of course, there is a hidden cost to that. If Killing Wave would have been an efficient creature, it might have filled a hole in your curve, put additional pressure on your opponent, or, in the case of black, helped you stabilize.
In the second case, you draw Killing Wave off the top. Because of the nature of AVR limited, there is almost never a situation of board parity — somebody is currently winning the race. This is the case where Killing Wave does not give you the luxury of carefully planning everything out, but you have to take things as they are. Of course, you might be able to swing with everything and fire off a lethal wave, but did this imply that you were on the offensive anyway? The more I think about situations involving Killing Wave, the more it appears to be a win-more card.

The Killing Wave argument ties in directly to the position of Black in Avacyn Restored. A defensive, card advantage oriented color with a powerful loner strategy is very diffcult to steer through a metagame of synergistic aggressive decks that can easily have you dead on turn 5. I admit that Killing Wave is probably a lot better in somewhat aggressive black decks, but up to now I have not seen a successful build of that. Or to put it differently, I have not seen a black deck make good use of Soulcage Fiend.

I disagree that it’s difficult to craft positive situations for Killing Wave, at least not in this format, and I don’t think it’s sensible to compare it directly to wrath, but I don’t think anybody’s seriously hurting their game by avoiding it. Especially if the consensus among good players is that a late Killing Wave isn’t a black signal.

I’m just trying to make the point that it’s a good tool if you know how to use it. Classifying a card like Killing Wave as “unplayable” doesn’t ruin your limited game or anything. It just cuts down your arsenal of weapons by one.

Killing Wave is “unplayable” because situations are very difficult to craft (much more so then with actual wraths, which can be tough at time in limited themselves) and arise organically relatively rarely so there are going to be fairly few times when killing wave is going to be a great top deck.

TBH this is probably a relatively “high value” format for killing wave to exist in because soul bond can create dramatic board swings, but if you must draft it don’t draft it high.