In March 1983, S.W.A.T. magazine published an article titled "How Close Is Too Close" by an instructor from the Salt Lake City Police Department named Dennis Tueller. That article discussed the research that Tueller had conducted to answer a question posed by one of his students, in essence, "How close can you let someone armed with a contact-distance weapon get before you can no longer effectively draw and fire your sidearm?"
Although the results of Tueller's research involved as much coincidence as diligent experimentation, they nevertheless pointed out an incredibly important maxim of close-quarters combat: If an attacker armed with a contact weapon begins his attack from a distance of 21 feet or less, standing your ground and going to guns will almost guarantee that the attacker will hit his target.
Tueller's groundbreaking research became the inspiration for the often-repeated "Tueller Drill," which basically replicates his initial experiments and validates how quickly an attacker can close the distance on you. Unfortunately, to some trainers it also gave rise to the term "21-foot rule."
And at that point, the real meaning of Tueller's research began to get lost.
What the Tueller Drill really teaches is that drawing and shooting alone are not going to save your life in a close-range encounter. In fact, the entire concept of going for your gun as an initial reaction is probably not the best way of ensuring your survival. Movement, the use of obstacles, and sound empty-hand tactics—applied properly—can all do more to keep you alive than a pure "gun-fu" response.
Article here. While one may argue with the specifics of the techniques you should use, the need for some form of empty hands / defensive tactics response to a sudden knife attack is indeed essential for successfully defending against such an attack.