I once wanted to be a police officer, and went through police training in Florida. I also currently study Aikido and Kali (Eskrima). I have practiced many knife disarm techniques and some defensive/offensive knife use. Taking a knife from someone who is actively trying to hurt or kill you with it and has even minimal training in how to use it for that purpose is extremely difficult. You will almost certainly be cut, and a cut to the arm can disable your hand. A baton improves the odds, but it's still very risky. Most police body armor does not provide significant protection from knives, nor does it cover the arms, legs or neck, which are excellent targets for a knife.

Disarming or disabling a knife-wielding attacker requires tools that can be used from a long distance. Pepper spray and Tasers are good less-lethal options, especially when multiple officers are present. These should be tried in most situations before going to a firearm, but if a suspect charges an officer with a knife there isn't time to wait and see. The range of a Taser happens to be the same (21 feet) as the distance most people can cover in the amount of time it takes for most officers to draw and fire a pistol. If the Taser fails or misses, it doesn't hold a second shot and can't be reloaded quickly. Most of the time, I'd want the officer to risk trying the Taser, but it's not reasonable to ask someone to go up against a knife unarmed.