User blog comment:Bobbs3k/Can Ichigo's New Bankai be stolen?/@comment-5551986-20130630190501/@comment-3042759-20130702074827

A bankai that has been stolen is incomplete without the shinigami who properly owns it to wield it. The sword and owner are two halves of the same coin. Neither is complete in power and ability without the other, whether in shikai or in bankai. The quincy may be in possession of the sword, but without the owner, the sword cannot be focused, and its edge is dulled as a result. It is still powerful enough to overwhelm the owner, but insufficient against someone who still holds their sword intact.