I don't think it's necessarily a problem with the "is this a serious threat?" evaluation, or with blood spells needing to be more liberal with blood (they already seem very eager to kill all their blood slaves, and I don't want that to happen when an enemy sends one silly unit against my army).
The real issue would seem to be that Hellbind Heart should be an exception to the gem use override, because you don't just want to win - you want to capture.
I'm wondering if this was remedied in 2.13, or if it will be in 2.14? Hellbind Heart (and a very few other spells) the AI shouldn't override just because it's "smarter".
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Wormwood and wine, and the bitter taste of ashes.