Yes, daily, on and off duty.
As has been said before, your gun or gun hand has a good chance of being shot in a fight, you may not be able to get to your primary, or it may have broken on you.
Another reason I carry an extra is the ability to arm someone who doesn't have their own gun. I know some will say that this person should have been carrying, but they don't always have a choice.
Example; I was at lunch with two guys who were military spec ops vets, more qualified to carry a gun than anyone else I know of, including me, but they didn't qualify for a carry permit where we were, and I was the cop in the group so I was the only person with handguns. If something had happened I would have been in pretty good shape handing off my BUG to one of them to assist me.
I also trust my daughters enough to arm one of them if things are really bad, but they aren't yet old enough to get a CCH license.