User blog comment:Xilinoc/Ch. 580 - THE LIGHT/@comment-24284608-20140517134349/@comment-5398287-20140517185716

My belief exactly. In the first invasion, Kenpachi killed THREE Sternritter in rapid succession with only his Shikai. Up until this point, every other Shinigami (sans Yamamoto) who has fought a Sternritter has needed a Bankai in order to win. However, Kenpachi proved that he's basically a walking Bankai BEFORE he got his huge power up from fighting Unohana and learned how to release his true Shikai. There's also the matter that even with the Tenshintai available, he wouldn't have been able to attain Bankai in the usual three-day time period because it's only been, like, 2 days since he first learned his sword's name.

So yeah, I honestly don't get why people want him to have a Bankai when it would be the most gratuitous thing to ever come out of this series.

And don't even get me started on Yachiru somehow having a Bankai.