I don't really mind extra materials being told in other mediums. After all it's a way to keep expanding the verse as well as earning money for both the author and the ones doing the other mediums without to much work for the author, which is understandable for mangaka since their job is very stressful and hard (you can read Bakuman as well as Google-sama to get some idea how that life is). As long as there's a good reason why the extra materials are not necessary or not possible to be told in the main story.
From a reader's perspective I'd love to know everything about a verse I love and prefer if the author can pull everything off, that would be best scenario. But if the story is told through certain point of view or certain purpose then it's obvious some info is not known or not important. Like Isane's Zanpakutou's ability for example. I'd love to know what it does. But she never needed to use it so...it's also not very important so logically Kubo can omitted it. Revealing it through mobile game is a good way.
Shunsui's bankai would do more harm than good no? With all the allies around and such.
Actually Aizen solo'ed them all and only he inflicted wounds on others LOL Prior to that only Shinji hurt him so if Shunsui activated bankai at best only Shinji got caught in it. Maybe there's some untold bromance between Shinji and Shunsui that we don't know about
Hell, if Shunsui's bankai has a time limit in terms of how long the period between the wound being inflicted and the bankai being activated then Shunsui should have activated it right after Aizen stomped Komamura or something...just gave the others some head-up
And Yama's was sealed by Wonderweiss right?
That was after he tried to sacrifice himself with Aizen. Ergo, no, he never intended to use bankai to begin with but opted to burn himself and Aizen with those pillars of fire from his shikai.
PlasmaWolf used to write a big theory about why the captains didn't use bankai vs Aizen though. You could find it in archive. It's pretty well-thought.
Rumor: volume 3 of the novel will be out on December 4th and Kubo will draw a sketch of Shinji's bankai. The bankai will also make its way to the mobile game Brave Souls so just wait for youtube video.