As mentioned it's also a great way to wear or damage the internals and produce a "loose" gun in short order.
What the two handed cowboy shooters are doing these days is "slip hammer" firing the guns. They draw and as the gun comes up they pull the trigger and pull back the hammer with the support hand thumb. When the gun is on target they just let go of the hammer. Repeat for the next four shots by simply pulling the hammer back fully and let go as the gun swings to each target. It's not hard on the guns and it more easily ensured that you get a full advancement of the cylinder.
Check You Tube for the faster cowboy action shooters and you'll soon see that this is still a fast and obviously far more accurate way to rapidly shoot a single action than fanning the hammer.