User blog comment:Xilinoc/Ch. 599 - Too Early To Win, Too Late To Know/@comment-4822693-20141004225027/@comment-4822693-20141006060016

I figure that Yawach has kept them out of the fight so far because he was saving them for the most powerful of the shinigami, and that there wouldn't have been much point in them fighting so far since their Vollstandig would be more powerful than a stolen bankai. That way they could be kept in reserved for the final series of battles.