Indeed Tsunade was useful when it first was implemented to us but after a while when you start filling your lineup with more advanced ninjas I literally forget sometimes the NPC actually exists. I think it could be useful again if it'd also offer books of the recent BTs that got implemented.
In china you can directly buy skill books for about 2k coupons, and you upgrade blue skills to golden with that
Until we get that implemented I wouldn't mind getting some new books in there!