Kakashi Is the Exception That Explains the Rule
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Transplanted Mangekyō · No Uchiha Body · Kamui Costs More to Sustain
Kakashi can use Obito's Mangekyō because the transplanted eye retains its ocular ability, but his body is not naturally built around the Sharingan. The result is a version of Mangekyō use defined by unusually severe chakra cost and limited endurance. During the war, the brief return of both of Obito's Mangekyō and his chakra allows Kakashi to demonstrate what the complete pair can do at a level he could never sustain with the single transplanted eye alone.
Why it matters
Kakashi separates possession of a Mangekyō technique from being an Uchiha. The eye carries the ability, but the body using it still determines how practical that power is.