Well simply put a safety is just another delay in putting out a round in times when your life is on the line. Ideally it is almost instantaneous but in a crisis you have to remember and have time to switch in off. Nothing can compete with pulling the gun ot and pulling the trigger.
Some say "well just don't use it". To me that is even more dangerous because you don't know if it got flipped on somehow in the holster. My policy is better if it has none but if it does I use it.