Board Thread:General Discussion/@comment-4976518-20140808063123/@comment-28044177-20170111184648

Cocoport wrote: Maybe Aizen never released his bankaï because it's a useless one, unsuitable one or it's overly dangerous for the user.

For example sui-feng Bankai is not suitable for her. And Rukia's is really dangerous because if she make a mistake, she has high chance of dying.

The Soi Fon case was because she didn't have a good relationship with her Zanpakutou. Something that changed after the tiemskip, since she no longer has those major drawbacks. And the Rukia case was because she hadn't mastered her Bankai since she JUST achieved it.