I always carry a knife; not so much as a back-up weapon, but because they always come in handy. I end up using it for something at least 9 out of 10 days.
A couple of knives I regularly carry could easily be used for self defense (Kershaw Boa, Sog Flash Tanto)
On occasion I'll carry a Colt Commander boot knife (which I would consider an actual fighting knife)
Usually (until I lost it a couple of months ago
after about five or six years and THOUSANDS of times being used) I carry a Kershaw Scallion. It's blade is 2 1/4"; it could really hurt you, but definitely not meant for self defense.
Everyone should carry a knife at all times. IMHO it's absolutely the most useful tool on the planet.
After all, our ancestors were using blades for cutting over 2
Million yrs ago.