The duration that Ichigo can use his bankai is absurd, and in terms of progression of power, it doesn't seem to be that great, it's essenitally comparable to the boost that other shimigami get when they use shikai. The amount of time Ichigo put into obtaining Mugetsu seems genuinely akin to the amount of time other shimigami put into obtaining their bankais. It's a final form, something else comparable to shimigami's bankais. The reason why it could only be used once is possibly because ichigio is truly human, if he were actually a shimigami, perhaps he'd have the durability to use it more like a bankai. At this point in the series it would make no sense for there to be a "third" release or final ("sword/shimigami fusion") ability that no other shimigami would have mentioned, or that only one shimigami has. That's the only reason why I'm questioning the terminology.
Achievement of the Shikai is a mark of control of a Zanpakutō, and it is a requirement for advancement within the Shinigami ranks, for most seated officers are capable of Shikai. It is required in order to achieve the rank of lieutenant, for it is a requirement which most captains look for.
Achievement of the Shikai is a mark of control of a Zanpakutō, and it appears to be a requirement for advancement within the Shinigami ranks, as presumably most seated officers are capable of Shikai. It is also required in order to achieve the rank of lieutenant; as it is a requirement most captains look for. After first learning the Zanpakutō's name, the Shikai can be performed at will simply by speaking a command followed by the Zanpakutō's name. The commands vary between users and range from a simple imperative verb to a short poem. They often relate to the Zanpakutō's signature ability, or hint at the nature of its spirit. This step may be bypassed by expert Shinigami who have learned how to use their Bankai.
Kubo may well be sick of Gin getting the , and decided to show Gin's fans once and for all that he is evil (since Turn Back the Pendulum didn't do it). What better way to prove this than to kill off the only two people from the Gotei 13 that he still supposedly cares about. Doesn't explain why the rubble from Shinso's Bankai didn't just crush them (and instead must have intentionally avoided them), but this theory needed to be tossed out there. The alternative is that Aizen will kill them, hitting Gin's supposed . Either is sure to be a , and make me laugh.