America has a strong cultural prejudice against the use of knives. Think about the action heroes we watched as a child on television and the movies, the hero almost never used a knife, and if he were attacked by a man with a knife, he simply disarmed him. The knife was portrayed as the tool of the weak, and the evil, and I think the stigma lingers.
If you have the time and the distance to make the draw, I can't imagine anyone taking the position that the knife would be the better tool, but at grappling distances a knife could save your life in a situation where you couldn't reach your gun.
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