Wouldn't it be cool if Yamamoto just slaughter Aizen when Aizen was using kyoka Suigetsu like he did to imitate Momo? Aizen is suppose to make a comeback, and he has the ability to pull off a switch like this without being noticed.
Speaking of Aizen, what the hell was Yamamoto doing when he fought Aizen and didn't use his Bankai? Sure WW would have sealed it, but it seems like Yama could have used his bankai from the start and save the captains a beating. Obviously that would have ruined the point of Kubo's story and all, but what the hell?
Digression aside, Juha Bach clearly had nothing to stand up to Yamamoto and so he needed to draw out Yamamoto's bankai, if that wasn't really the enemy leader (yet he still beat Kenpachi with no effort) Yamamoto unveiled his true powers to a fake. Next chapter, the real Juha Bach pops out of the ground and seals Yamamoto's bankai, kills him, brings him back to life, and thus the royal guard and Ichigo become relevant again.
Just some speculations from a fan, but this sure is one hell of an arc, so much so it makes up for that boring and anti-climatic fullbringer arc we endured =)