I'm only going to say that if someone is trying to disarm you then it's a deadly force encounter. You may use a knife in a deadly force encounter.A couple more things to consider...
When I was with PMO on Camp Lejeune, we did not train for knife use. Because it was not considered an acceptable secondary or less lethal weapon. Simply put, in military LE, cutting someone is bad and falls outside of justifiable force.
And during weapons retention training, we were taught, as mentioned by George, two hands are very obviously stronger than one.
This also puts the offside arm in a semi defensive posture, covering part of the chest and abdomen in the event of an incoming strike from an attacker, and makes it more readily possible to deliver a counter strike with an elbow or shove with the shoulder.
Versus leaving your front wide open, while trying to retain your most lethal weapon with only one hand? To retrieve a secondary weapon, which the use of I've never heard of being approved by any department? And that secondary (knife) being smaller and inherently trickier to grasp than a baton?
None of that makes sense to me. But hey, I'm no longer in law enforcement and haven't been for a while, and have never been in civilian law enforcement.
Come if you must, but only if you must. For the day you find yourself upon my step shall be the night you find peace along Jordan's edge.