Ok, since this thread is still going, I'll editorialize a little.
It's true that if the shooting ends (or never gets started) and things turn into hand-to-hand, having a knife will be handy. However, if the confrontation turns out to be a gunfight it's best to maximize your effectiveness as a gunfighter. Holding something unrelated to gunfighting in your off hand instead of using your off hand to improve your accuracy and speed your followup shots isn't really a great strategy.
Having a knife handy is smart, as is having a flashlight. However, carrying the knife around in your hand the whole time you're in a defensive encounter even when there's no need, makes as much sense as carrying your flashlight around in your hand the whole time in bright sunlight.
Having it readily available makes a lot more sense than pre-strategizing that you will always be holding it in your hand.