Board Thread:General Discussion/@comment-24970687-20140531231739/@comment-1268838-20140601103101

The power of Ennetsu Jigoku alone was sufficient to completely decimate the Fake Karakura Town, and the barrier surrounding it along with it. The flames produced by that ability were sealed within Wonderweiss, while the rest were trapped within Ryujin Jakka's blade. Had not Yamamoto not absorbed the brunt of the explosion, Kagamino City would probably have been caught in the blast radius, killing hundreds if not thousands of people.

Point being, if his Shikai is capable of completely nuking the battlefield, then his Bankai would have had an absolutely catastrophic effect on the Human World. We already know that it's capable of destroying Soul Society from its ambient heat alone, and chances are, the Human World is not nearly as resilient.

It's true that defeating Aizen was a high priority, even to the point that Yamamoto was willing to sacrifice the entire Gotei 13 to do it. But when it comes right down to it, protecting the Human World came first for the Shinigami.