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The Grail doesn't judge the candidates on their skills but rather their value as beings in fact (hence why Kirei regained his command spells).
To begin with, as explained before, Kariya is no real magus: he doesn't have any magic circuit and turned his back to the Matou, so having a makeshift magic circuit with the crest worms is only good enough to be a "battery" for Berserker, although at a steep price.
And it is exactly despite this huge handicap, hardship but still a strong conviction and will that kirei took interest in him.
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