Notorious, that is actually a very good point. A knife can be used to shield your gun to prevent someone from grabbing for it. A blade is something that is very difficult to take away from someone barehanded, far more so than a firearm, because you cannot grab hold of the exposed part of a blade. Effectively it is a physical contact deterrence.
In the past I carried a Leatherman with me at all times. This particular Leatherman has an external locking blade approximately 4" in length (and as many people know, new Leatherman blades are razor sharp). If anyone asks, it's a tool, and its other functions can easily justify this. What most people do not know, however, is that I always sheathe it in a very particular way such that, when drawn from the left hip, the knife blade is always on top and to the right. This means that I can quickly draw it with my left hand and flip the blade open with my right in one smooth motion. The knife is always available and can be deployed without even looking at it.