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Unironically. Even if Medea could never do Shirou's magic. Wouldn't she, unlike Rin, actually have a good chance of actually understanding Shirou's magic well enough to help Shirou through the process of mastering his magic?
And if she could strengthen him with her super buffs, and supply him with magic that she could absorb from leylines and stuff in the city, allowing Shirou to actually keep his Noble Phantasms and UBW going. Wouldn't that be way more efficient for both of them than any other Master / Servant combination?