If you fire at someone and hit their weapon it should fly out of their hand and be dropped to the ground. The weapon on the ground could then be picked up again.
I'm thinking this should only apply if you shoot at someone from the side and not head-on because then everyone would drop their weapon all the time. So if you see someone standing behind a corner and only exposing their weapon you could shoot it out of their hands.
Not necessary before we have an option to brush our teeth too (not all things should be in Arma).
Is this possible in real life? Ya, kind of... Does it happen often? No, not really... What happens when you try to pick up the weapon in Arma? You try to find the right spot for 2 seconds? You scroll through your action menu for 2 seconds? You try to drag the item ot its right slot for 2 seconds. Now compare that to the real-life situation. This is why this shouldn't be in Arma; it's not possible to simulate it properly.