My wife carries OC.
We have a can of Bear Spray for answering the door (even if other arms are neglected) -- fog*. OC in your hand does't frighten the UPS guy so bad.
Spares OC in both cars.
I carry OC.
It gives me another option -- most people will actually wait TOO LATE too shoot -- OC is easier to decide to unleash and pretty much a 1-shot stop* when it works.

It frequently legal even when a restricted area forces disarmament.
I carry 2 decent, lockable pocket knives, mounted on each front pocket
(same reason, and they are mostly used for utility purposes -- also, hard to cut a jammed seatbelt with a Glock.)
I always carry a cell phone -- you will use the phone for 100 emergencies for every one you need the firearm, if you need the firearm you will definitely need the phone.
I carry and give "Fog" style which:
- You can bring it out early -- in your hand -- even when deadly force is not authorized or just as a precaution when vulnerable but no actual threat is ID'd -- Condition yellow++ or orange
- Requires NO real aiming nor training
- Legal or at least less illegal when deadly force is not authorized
- Good for when an threat needs stopping but you really don't want to shoot if (for legal, personal, or any other reason) -- like a sloppy drunk who is dangerous but only stumbling or flailing around
- Good for multiple assailants
- Probably will look better if a big tough guy OCs a tiny but dangerous woman (kid, dog, etc) than if he shoots or punches her
- Works even if you're already in someone's grasp (it you can get to the trigger)
- Works to lay down a blockade for retreat (either in the open or in a retail store where you head for the back etc)
- Is pretty much a doomsday device if you are inside and hold down on it -- everybody's gonna get stoned.
Did I mention that I like OC? But if I could only carry one item it wouldn't be the OC.

Fourth point of contact! I like that. Airborne!