User blog comment:Godisme/The Nine Demon Gates/@comment-999343-20131027002215/@comment-7418318-20131027080250

There is the psychological effect of taking away the enemies powers and turning them against their former owners.

Or since Bach expected the Bankais to be taken back, maybe he has some "evil master scheme" up his sleeve... that will end up being completely ridiculous and also bite him in his own ass, just like Aizens stupid schemes to help Ichigo grow stonger came cave back in on him.