Most of the time when we disarm someone in class, the weapon goes flying across the room. Yes, it would be out of play. Especially if it happens outside in the bushes. A lot of the motions we use to disarm someone would break their arm as well. You're under some sort of flawed assumption that the idea is "take the knife from the opponent, place it on the ground, and then taunt him." No. You continue to fight, using the martial arts you've already trained. If you are between your enemy and the weapon, he is either going to have to dance around you or go through you. If the weapon is away from you (especially a knife) that gives you a chance to run while he's going to get it. There's a lot of if's and's or but's, but the thing is if you get the weapon away from your attacker, it's not something he's going to immediately have access to. And your fighting training applies at that point.