Firearms instructor John Farnam writes about training experiences with particular pistols (with a follow-up article here). There are certain pistols that have external hammers but those pistols must be carried with the hammer decocked, such as the SIG 226, Beretta’s, H&K’s, etc.; to carry with them cocked is dangerous. What happened was experienced shooters were reholstering their cocked guns and didn’t realize it until an instructor pointed it out.
If well-trained people can forget….
IMHO, decockers add complexity. If I think about it from an engineering design standpoint, and more specifically thinking about the “user interface” to a pistol, decockers are a design feature that is more complex than it needs to be. Granted that’s part of the point, when you consider the evolution of where these guns came from, but it no longer has to be the case. When you have guns like Glocks, Smith & Wesson M&P, Springfield XD, and so on, they provide a much simpler user interface. You’ll also see that other manufacturers are moving towards similar designs. Evolution.
Read my write-up on choosing a gun for more details.