For a 1911, use it. The reason you can get away from a manually engaged safety on a glock, m9 or HK is that they have relatively long and heavier trigger pulls to reach the break point on the sear in comparison to 1911s.
Couple the short trigger travel and the light weights people put on their 1911's and you're asking for trouble without using the thumb safety.
It all comes down to a training issue. Thumb on thumb safety when establishing the grip in the draw, it doesn't get flicked to the off position until pressing out (sights on target) from the high compressed ready. After engaging, it gets put back on safe after coming to high compressed ready before you scan.
No matter what the gun... Safety on while carried. It doesn't take you any more time to take it off when needed. If you can pull the trigger under stress, you can certainly manipulate the safety on and off as needed.
For the record, grip safetys sre dumb.I pin my grip safetys on the 1911, and only leave the thumb active.