@Razh:
Reading that interview about Kubo's creative process and last minute changes, it's safe to assume that, if a character hasn't used their Bankai yet, it just means Kubo hasn't decided what to do with it yet.
Shunsui… Ukitake: Your Bankai shouldn't be used here/ isn't suited for this fight / whatever... Lol, gimme a break.
Ukitake...............
Hisagi... Can't get enough panels space to finally use his new Bankai, since Kubo doesn't know about continuity.
Shinji... Seriously, can't stay in a fight long enough to even say the word Bankai from start to finish.
Unohana... Hid her Bankai for ages only to reveal that it's shit and something Kubo most likely thought of in the last minute.
Who knows, maybe he doesn't even know what Ichigo's Bankai will look like, yet.
Worse part is, not having their Bankai's planned out isn't necessarily a bad thing, but with the lack of any type of planning for the series, nobody (except his fans) really has confidence that he will deliver. The problem as well is that with all the bankai's that still need to be shown, they'll either be like the Vizards, who use the Bankai and get owned right away, or they will get revealed to be broken because the enemy dictates a needed power to win. In the end, it will be disappointing, because there are so many people who need to show of their abilities still, and to few enemies.