It's the draw of the one gun man. The man who doesn't need a lot because he's so darn good with what he has. Like the Duke with his ivory handle Colt. He didn't need a two gun rig, or other guns stuck in his belt. One was good enough for the Duke.
Two very different narratives, both of which have tremendous truth and staying power.
1) Choose the right tool for the job.
2) Choose a few good tools and learn how to use them for the widest range of needs possible.
I see this same exact discussion played out constantly in bike and ski forums. I buddy of mine is a carpenter and big into the hand tool scene and says the same exact debate is being played out there.
I think one knife for all things just doesn't hold up. A sub 3" bladed pocket knife is one thing. A >4" knife is another. I can be at peace with a smaller pocket knife if and only if I know I have access to a larger knife when I need it. Food prep is one place I want a bigger knife. Heavy yard work, fish cleaning, camping.... these are places where I want a bigger knife available.
Interestingly, now that I have an Opinel #10 tucked away in my PDA pouch (or in my day pack - depending), my 3" lockers are getting a lot less carry. When I didn't have the bigger #10 around, sub 3" pocket knives just weren't enough.
I wonder a lot about the hey day of the pocket knife and the demise of the sheath knife and wonder if they are related. Did the small pocket knife make more sense when sheath knives were more common? My smaller pocket knives make a lot more sense to me with a good 4" blade nearby.
As for Duke and his pistol, I think he also generally had a Winchester lever action in his saddle holster, so we can't say he was really a one gun guy.