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I believe this mastery should be put into the defensive tree. Off of the top of my head, 3 champions stack a large amount of Mana: Yorick, Singed and Ryze. All three of these champions go down the defensive tree as it is the most effective, and there is no reason for them to go down the Utility tree besides the now Strength of Spirit.
Also, the only people who regularly go down the Utility tree are *drum roll* supports. Supports do not use Strength of Spirit very effectively because they very rarely have any items that give them significant Mana pools.
Thus, I see no reason why Strength of Spirit is in the Utility tree.