Well it is definitely useful, it's just that as the DO's apparent lone kryptonite there needs to be a reason either why he had hand it out or alternatively can't prevent its use. Like it needs to be used to weaken the pattern and free him or with the bore open anyone can access it same as if it were oil spilling out of pressurized well. Because otherwise, no matter how useful it is to his followers, he'd be dumb to give access to anyone. I'd guess on the latter option since my impression is that the DO relies on balefire - regardless of its power source - to weaken the pattern and that he needs it to be just damaged enough but not too much when his prison is weak enough so he can both escape and actually have something to escape to. Otherwise he would have made his AoL followers keep using balefire and as much as of it as feasible.