As you know, you can pull the trigger part of the way back and cock the hammer the rest of the way with your thumb. I think that is why they put the serrations on the hammer at the top. It is dangerous but if you are just plinking, it is not bad. Why would you need to single action it in a defensive situation anyway?
I know what you mean and I would like the option as well. It is the same reason I want a 638 to replace my 642. I want to know if there is a hammer available as well.