Ukitake said to Shunsui when he was fighting Starrk to not release his bankai where others can see. He clearly did not need it against Starrk. As for why neither of them used it against Aizen, it is possible that it is simply not possible for the others to fight together against Aizen like they did if he used it. They could just end up tripping up over/interfering with each other. —Preceding unsigned comment added by Yyp (talk • contribs)