It's not that useless, it's just that it didn't work on the inner Noah (I think it's too evil to be exorcised). Also he doesn't want to hurt humans right? So It'll be pointless for him to go back to his "claw" mode.
I think that was just in the moment that Allen said he would kill the noah. However I guess if you drive him to that point maybe his sword will get deadlier for once. To tell you the truth that power up probably only works on Akuma its useless in a fight. Allen might as well go back to Crown Clown mode.