Definitely roll the spirit's dice to resist, using your house rule, in order to compute drain; then absolutely make him roll drain. Because you see - if the drain knocks him out, the spirit goes free. And then instead of having a kickass F12 spirit to stomp BBEG, they may be a) down one mage, and b) be facing a F12 spirit who wants nothing more than to turn Yon Unconscious Mage into paste, along with all his buddies.
Yay!