...people who carry a revolver for self defense should practice almost exclusively for double action fire, as if the single action option wasn't even there. Why? Because there are almost no situations in which single action fire is appropriate in self defense. Most self defense situations unfold rapidly. There isn't time to thumb cock a revolver and take careful aim in the way one would do while target shooting. A cocked revolver is dangerous in the adrenaline dump of a lethal force encounter. The trigger is just too light. Its too easy to fire when you don't mean to. There was a well-publicized case in Miami several years back in which a police officer accidentally shot a suspect he was holding at gunpoint with a cocked revolver. The suspect was killed and the officer faced a lengthy court process which ultimately destroyed his career.
In a nervous situation, a cocked revolver is dangerous. When you're really nervous or scared, the heavy double action trigger pull is an asset rather than a liability. I can hear you say, Keep your finger off the trigger until you're ready to fire, and that's true, but we also know that people don't always do what they're supposed to do in the stress of a deadly encounter. The police officer in Miami is a good example. I'm sure he had heard the rules. A firm double action trigger can be a welcome piece of insurance against an accidental discharge. With a DAO revolver, manual cocking isn't possible, nor is it possible to be accused of negligently cocking the hammer in a civil action which could follow a self defense shooting.