"The only use I see for it would be if you manually de-cocked it (i.e. lowered the hammer under your own control with the trigger depressed onto a loaded chamber)."
Right. That's how it's done.
Is there a question here?
Willie
Oh my god, no. Please read Wally's post. Never lower the hammer while the trigger is depressed.
I always use two hands. I use the strong hand thumb to block the hammer. I use the offhand to grip the slide, so that I do not rely on a partial, thumbless grip to control the gun while pulling and letting off the trigger.
To elaborate a little more on my own method of decocking:
1. grip gun in regular shooting grip, finger off trigger
2. point in safe direction
3. grip gun firmly by the slide in offhand
4. completely block the hammer by putting strong hand thumb over top of the hammer face and wedged between hammer face and slide.
5. make sure strong hand thumb is indeed in control of the hammer in addition to just blocking it by pulling back on the hammer until you can feel it start to float. Once satisfied you are sufficiently tensioned and braced for when the sear will be released, back off pressure a little so that in step 7, the hammer will come off the sear without any fiddling.
6. Doublecheck you are pointed in a safe direction
7. pull trigger momentarily and IMMEDIATELY let go the trigger and remove finger from triggerguard.
8. Now, carefully lower hammer. If the hammer does not lower because it is still on the sear, go back to step 5.
This applies to any gun with a firing pin block or a half cock notch. Gun won't ND if the trigger isn't pressed. So don't press the trigger any more than you need to. That split second you pull the trigger is the only dangerous part, if you do it like this. The other half dozen steps are there to make sure you are prepared for this moment. If you don't have at least a half dozen mental safety checks in your decocking process, you are negligent. Decocking a gun is not something that ever need be done in a hurry, under duress, or while having distractions. Therefore, it is not something that should ever become second nature or automatic.
CAUTION:
Some DA/SA guns have a halfcock notch, but they are not designed to be carried on half-cock. Bersa Thunder is an example I know of. If you used the decocker, the hammer is lowered all the way down. If you decock the gun manually, the hammer will be left on the half cock notch. Do not attempt to manually decock this type of gun.
As for flipping on the safety for "maneuvering," I think it's faster and safer to remove your trigger from the trigger guard. At any rate, you ought to do that before manipulating any controls on the gun. And at that point, putting the gun on safe while your finger is off the trigger and you have and plan to keep the gun in a shooting grip is kind of redundant on a modern, drop-safe handgun. The four universal gun safety rules don't mention manual safeties for a reason. The more stupid stuff you add, the less you are focusing on what is important. I realize this is just an opinion and there are LE who are trained to constantly flip safeties on/off. That might make sense for someone on a SWAT team that is trained to overtake criminals through surprise and force of numbers as part of an overpowering team of body-armored personnel. As a regular joe citizen, if I need to have a loaded gun in my hands, it's because I'm in imminent fear for my life, and I don't need any additional things to worry about. I'm not going to train to add more. The safety only goes back on before I reholster, put down the gun, or hand it to my shooting buddy.