The non-willing version is a zero-sum spell, unless the spell overcharges
It's ingredient is a drop of your own blood. I assume it's always obtainable. That gives it +1 Power Level compared to the WP you pour in, which means you always get atleast +1 WP from casting it.
I haven't really seen it as a way to stop enemies from casting, more of a way for me to get WP back.
Just wondering if you can even use it on NPCs after the revision, as they don't even appear to have WP anymore.
A lot of risks for little gain, if you are using the ingredient (sure, that is not hard to get), you get +1. In my eyes, that is a very high risk for very little gain. A normal sword fight, involving sword-fencing and dodging most often generates more than that and is less risky (if you are ok in sword-fencing-dodging that is).
The problem with the pool is that, how many WP does a single NPC have. If you as a GM have 10 WP in your pool, they don't all belong to one NPC, so why should the spell-caster be able to transfer all those to him. What if the GM have 40 WP in his pool. Can you capture one NPC, cast the spell a couple of times and transfer all those to the spell-caster and his friend? No, of course not, so using this version, the GM had to use a lot of GM discretion how many a single NPC had. I used 1-3-5 here, 1 for non-importart NPC, 3 for import NPCs and 5 for very important NPCs.
Using the new WP system. All non spell-casters are assumed to have 1 WP each turn and spell-casters are assumed to have rank (usually 1-3) each turn. So here we know how much you can steal (1 or 1-3). But here we have another problems. If allow using this in a friendly way, you can transfer 1 or 1-3 each time you cast the spell.
This is the reason, I almost never have my NPCs be willing to this, unless it is a special situation. They can cast it in combat and steal some WP. Here I just assume that the target NPC doesn't have his 1 or 1-3 each turn, that turn. Most likely not worth it because of that. Maybe as a pre-strike counterspell. If you face a sorcerer, cast the spell and he have to use something other than spell-casting that turn.
But, again, the best use of this spell is to transfer WP around the players, so the player that need them the most have them. Still risky though.