I think it's a good move, Bleach is a very character-oriented manga and Kubo likes giving panel time to most of the cast, unlike some other manga where secondary characters are left in the dust.
This obviously isn't liked by everyone, since it can affect the pacing negatively. But I don't think it will be as annoying as the Fake Karakura Town arc. Why? Because this is the last arc and this time Kubo doesn't have to make the characters hold back stupidly and act as if they hadn't a Bankai (like he did with Shinji, Shunsui, Ukitake, Aizen, Urahara, Isshin...). The bankai stealing medallions provide a good reason for characters to hold them back, and once this device is neutralized they will all show their full powers. It's the final arc, as I said.
This also gives characters who are heavily dependant on their bankai like Hitsugaya and Koma (Soifon barely uses it and Byakuya has kido and shunpo skills and will probably train at the royal palace) a chance to show that it's not all they can do.
And Hisagi getting a bankai, Renji improving his, and what else we might see later (Rukia getting her own bankai? Rangiku?) can indeed be considered fanservice but it's understandable since someone has to replace Yama, Unohana and other captains that might die later.