User blog comment:Xilinoc/Ch. 560 - Rages at Ringside/@comment-7079377-20131204201204/@comment-11051098-20131205231633

I partly disagree. While most characters who has Bankai are captain-level, I don't think that's obligatory.

Kenpachi Zaraki is a captain and has immense reiryoku, but only has a nameless full-time Shikai. Ikkaku has Bankai, but is a 3rd seat and isn't necessarily captain-level.