I've carried a knife (at least one) every day of my life since I was something like 5 years old.
Right now I usually carry three; two different pocket knives, and a Leatherman Wave tool.
None of my knives are designed or intended for SD use, although I suppose they could be used that way in a severe pinch.
My ideas for knife fighting are along the lines of:
Carry a gun for back-up.
If you need the knife, use the gun.
So, I carry (and usually use) knives daily, but they have little or nothing to do with the fact that I also carry a firearm at the same time. My knives get used pretty regularly for cutting tasks, evidenced by the fact that I sharpen them far more often than I clean my EDC handgun.