I've read a number of comments and opinions in this thread which seemed reasonable and with which I found myself nodding in general agreement.
The debate between DA & SA mode practicality for the J-frames offers much the same advantages/disadvantages as when full-size traditional double action revolvers are involved. The little J-frames come with their own added considerations, though, involving a smaller grip, shorter trigger reach, lighter weight (more felt recoil) and shorter sight radius (not to mention less refined sights).
Cops who carried revolvers as service weapons remember the significant majority of their training and shooting was done in DA mode.
A number of those of us who carried revolvers in that role can probably also remember occasional situations where a cop's revolver which was cocked into SA mode resulted in a ND in the stress of the moment, and a number of those instances may have also involved the tragic death of someone who was not supposed to be shot.
There was some understandable reasoning involved when some LE agencies eventually had their revolvers modified to DAO. FWIW, the CHP here in CA transitioned from revolvers to semiauto pistols back around '89, and they still order their M4006TSW's, as they did their previous M4006's, with spurless hammers which requires that the initial shot is fired in DA mode. There was reportedly specific reasoning for their choice to order their pistols with spurless hammers (the 4006 comes standard with a spurred hammer).
The use of SA mode in a revolver may benefit folks who enjoy target shooting, the ability to make a 'precision shot' (hunting, load development, etc) or an enjoyable 'plinking' session.
I like my older 649 (.38 Spl model) quite a bit. Its weight was the reason I originally ordered my first 642-1, though, and why of the 5 J-frames I presently own, 4 of them have alloy frames.
Oddly enough, those 4 alloy-framed guns are all DAO, too. I foolishly allowed a couple of excellent steel 36's to slip through my hands over the years, unfortunately.
Now, I grew up learning to shoot SA revolvers and SA pistols. It was with some small amount of skepticism that I finally picked up my first trigger-cocking wheelguns.
(Which was later revisited when I first started using traditional double action pistols, but that's another story.)
All of the courses of fire in which I train and qualify using my J-frames are done effectively and well when using DA. I also do some longer distance shooting with my J-frames to practice both my DA & SA shooting skills, but my main emphasis for defensive skills/tactics practice is done using DA. I like DA/DAO for defensive shooting training & practice.
I carry my 642-1's or my M&P 340 much more often than my 649 nowadays, simply because of how the lighter weight guns carry when used with my chosen carry methods.
I still like my 649 a lot, and someday I wouldn't mind picking up an Airweight Bodyguard (638) just to complement it.
In the hands of a skilled and experienced revolver shooter a J-frame can exhibit very surprising accuracy, DA or SA.
Most folks do find them harder to shoot accurately than larger size revolvers, though. Takes some work.
