However I don't agree with your view about if anyone grabbed your wrist when you're holding a knife that you would simply do whatever. I understand, I think, you get this from the view of Aikido not tracing back to days of the samurai and so you don't see the reason for practicing from that view maybe. ie: grabbing the wrist of someone holding a knife etc. (correct me if I'm wrong)
I do lot's of practice of doing this kind of thing don't you?
If I can escape from a wrist grab when I'm unarmed, why would it be different if I'm holding a knife? I can let him hold on and still cut his arm in various ways, but I'd also be moving in toward him with a view to the major targets of the body, the throat, the eyes, etc.
Of course aikido harks back to samurai days, but there is no precedent for nage having a knife against an unarmed attacker.
Now if uke has a knife, of course it's very dangerous, but nage's only hope (the same as with a sword attack) is to avoid with the body, parry the knife hand and seize with an inward or outward twisting motion leading to techniques such as yonkyo or shiho nage.
I'm just saying that in the demo Chris showed, exactly as he showed it, his grip would not prevent uke's slicing his arm up.