Hi! In my opinion, each ninja has their own use and it is dependent on the player whether the ninja works for their preferred play style right now or in the future.

Regarding the Konoha Great Tree, I think having Minato [Edo Tensei] fragments there is a way for players to obtain some of his fragments while using coupons/ingots on Konoha Great Tree, but I would not say it is the best way to obtain all his fragments. I will definitely forward your feedback regarding Konoha Great Tree =]

Regarding Minato [Edo Tensei] in general, if you are looking to redeem him, the Total Recharge Rebate is a good way to get him as you recharge and keep the resources (ingots) you recharged, while also getting Minato [Edo Tensei] fragments, Golden Treasure Tool Essence, and among many other items. There are very few ways to obtain him better than as a rebate, but there definitely is other ways to obtain him in the future.

Hope this helps! =]

~Jib