Holstering ANY gun, requires care and attention. Relying on another manual safety or two to do so, isnt a guarentee you wont have problems, especially if youre relying on things you may not always remember to set, especailly under stress.
Paying attention to what youre doing when you holster, regardless what the gun is, will always be safer than relying on something extra, that adds complexity and requires more from you to make the gun safe.
Proper and ingrained handling skills, are more important than adding safties. And even more so, when stress is involved.
Another thing I find intersting too is, when you draw something like a 1911, or any gun with a manual safety, the safety comes off as the gun clears the holster, so from that point on, what type of gun is in your hand, is pretty much a moot point.
DA guns, once fired, are in the same category. DAO guns are a little different, and probably the safest, especially in the hands of the unfamiliar, but they still need your full attention.
Im sure, if youve been around long enough, youve seen people do stupid and/or unfortunate things with all sorts of different guns. Ive seen people have AD's with 1911's that were supposed to be safe, Ive seen SIG's that were fired and reholstered without decocking, guns with safeties that were non functional from the factory, etc. Relying on things you have to set, or things you dont constantly vett, is just asking for trouble, and only a matter of time before somethng goes wrong. Constant proper training and practice will do more for you, than adding another safety.
But isnt it funny, how its always the guns fault, when something goes wrong? Its never going to be the users fault, because..... well, you know.