It is useful for a quick kill... because at least as far as we've seen, if you can land the thing, your target suddenly has no defenses and you can mulch them pretty quickly. Still, if it is not the thing we are doing now, it is not the thing we are doing now.
Personally, I mostly want to find out what the boon is.
We out-dice this guy by quite a lot in all likelihood just personally and also outnumber him, with a lot of us attacking, meaning that spending an action (and all the dice inherent in that) to remove his ability to defend just isn't super necessary...we're gonna hit with almost all our attacks anyway. It's useful on somebody who has enough dice that making them unable to use those dice is relevant. That doesn't seem to be the situation here.
Like, in order to have a Knee-Groin Trick go through when a normal attack wouldn't, we'd need to give it more dice and thus reduce our total number of attacks, but this guy is getting attacked with at least four attacks (us, Aki, and both archers) before we even start in with our 'low die' attacks (at 5d, they're actually more 'mid die' by reasonable standards). Given his likely dice pool, either his defenses are gonna be at low enough dice that our existing attacks get through them, or he's gonna run out of defenses and they'll get through that way, meaning that Knee-Groin Trick's best advantage is just irrelevant here.
Knee-Groin Trick is very good if you use it early on before a lot of other attacks, and then use the opening provided to do high damage attacks, all especially true if you're fighting one-on-one or you're the one outnumbered. And that's a solid tactic we may play with later, but we don't need to make an opening on a guy who's just straight up getting overwhelmed by attack volume already, and it would honestly likely slow down how quickly we killed him.
It's great to set-up a finishing strike, but not really a viable opener.
It's a viable opener on a single opponent you don't outnumber if you throw enough dice at it, IMO. Well, a viable second attack after they lose their shield, anyway (or even a first attack if they don't have one). This just isn't that situation.