I actually teach and practice some techniques around this idea. I've been doing Martial Arts for 11 years and specialize in edged weapons.
I think what some people don't see is that having the knife is really just to assist in retention of the gun. This ISN'T a "knife fight", this isn't a situation to rely on the knife to kill your opponent. The knife is used to gain distance if the opponent reaches for the gun. Now, in the fashion I use it, the knife is in an "icepick" grip, edge in. This creates a hook with an edge that is mostly used to pry down and against the direction of the opponents wrist. Using downward pressure is easier for weaker opponents, and going against the skeletal structure is more important than the wound created by the edge. To be careful, the gun is swept down to angle it away from the opponent and put the knife forward. Gain some space and get the gun up. I include other techniques to cause more lethal wounds, but mostly for those who want to learn them. I also prefer to use knives that have a slim enough handle that they can be held while both hands are on the gun.
Again, there is the disclaimer. Pain doesn't always work for compliance, some people won't even notice a cut or stab until it's too late. Being that close to your opponent when you cut them increases your risk of contracting a blood born disease. Techniques like I talked about require some skill, which means practice, and no hesitation. Reaction time is important, having no reservations about using the knife is just as important. Using a knife is not like pulling a trigger, anything you scrape will be felt through the knife, so mindset is important. There is no perfect opponent who allows the technique to work right, there's a chance they will throw punched or low kicks, this is something to be dealt with.
"Today is victory over yourself of yesterday; tomorrow is your victory over lesser men." - Miyamoto Musashi
[Insert random irrelevant religious quote here]