User blog comment:CoolJazzman/Why do you think Yamamoto wasn't giving a promoted to become a royal guard./@comment-8295743-20130417035134/@comment-6904925-20130418063816

I'm not sure why Jushiro, Isshin, Rose and Love didn't release their bankai's in their battle against Aizen, but for Yamamoto he needed to feel the Spiritual Pressure of his sword to know or not if that was the real Aizen and not an illusion thus with his bankai west ability, the one where he wears a suit of fire is automatically activated the moment he released his bankai, thus he wouldn't been able to trap Aizen. Kisuke mentioned his Banaki is not the kind to "help others" so I think he was concerned about Isshin and Yoruichi if caught within it or something. Shunsui bankai, Jushiro its not something others should see, so it might the kind that affects both friend and foe. Shinji on the other hand, it was told that his Shikai is his "trump card", so maybe either his Bankai is something his not proud of or like Shunsui and Kisuke it effects both friend and foe at the same time.