system
Given the following post, respond as if you're commenting on Reddit:
human
[Bleach] Why didn't Aizen just use his "Bankai" instead of turning into that giant butterfly thingy?
gpt
A Bankai is only a five to ten time boost to your spiritual power at the outside.  The Hogyoku was in the midst of transforming Aizen into a completely new being with untold powers.  So while Aizen absolutely *has* a Bankai, or did at one point, he has so much more power than Shinigami did beforehand (although its debatable if he really had much more power than Yamamoto given the need for Wonderweiss,) Aizen wanted a different path.

Think about how Ichigo had to fight Aizen.  Ichigo was about the same strength as a Captain at the end of the first arc, and had his own Bankai that Aizen stopped with his finger.  Then later, Ichigo's grown much stronger, and his full power into an attack didn't even scratch Aizen.  Yet Aizen saw there was the potential for even more power than a hollowification through the Hogyoku.  Unfortunately he was wrong, as Ichigo's combination with his Zanpaktou for the Final Getsuga form proved, Aizen was pretty much the king.