Would anyone carry a revolver with an empty cylinder?
It is still done using older design revolvers without hammer blocks or rebounding hammers.
Early and original Colt Peacemakers, early production Ruger Blackhawks, almost all black powder revolvers and cheap .22 revolvers should still be done that way.
Bumping the hammer on these with a round under the hammer will cause a Barney Fife.
There is a fear among some revolver owners of dropping the revolver and having a round go off. So they carry the revolver with the hammer on an empty cylinder. As for a semi-auto, I guess the same reasoning would apply. Someone might be afraid that a round would go off if the gun were dropped. The main problem with carrying a semi-auto with no round in the chamber would be that if someone needed to fire, and had almost time, they would have to rack a round in the chamber, and a small delay could be fatal to them. Zimmerman’s life hung on his ability to get one round off. Had he needed to rack a round in the chamber, he might be dead now.
With some guns.
The cylinder under the hammer will actually not be the cylinder fired on the next trigger-pull. It will actually be fired last.
Some of the early Ruger Blackhawks did not have a hammer-block safety, and if you dropped it on the hammer it could fire if there was a round in the cylinder.
The workaround was to leave the cylinder under the hammer empty until you could send it in and have Ruger upgrade it to a hammer-block.