Honestly I can't think of any instance where I'm being shot at or possibly going to be shot at that I wouldn't want to be wearing a PC and brain bucket. Hell brother SHTF and I'm going to be wearing that shit when I'm more than 25 meters from the house.
I'll try and break it down and if this isn't clear enough let me know - I have a bad habit of going into acronym soup sometimes. Let's look at it from 2 viewpoints - the aggressor and the defender.
Our current aggressor rarely ever wears body armor (seen it but it's pretty rare). When the aggressor stands and fights he normally dies. If his buddy is hit he normally ends up leaving him to bleed out if he isn't outright dead while he tries to make his escape only retrieving him if he absolutely thinks he can and survive. When he attacks and fades almost instantly back into the population he stands a better chance at getting away but there are things like handheld residue and gunfire direction detectors now that make it harder. When we first went in and got hit inside of towns it wasn't uncommon to chase a bastard down a street and have him turn into an alley. When ya got there all you found was a rifle laying on the ground - maybe if you were lucky you might catch the guy but I saw a lot of them get away. By using remote attacks (IEDs, mortars, sniping) his success vs. loss rate is a little better for him. Overall from what I have seen if we look at pure numbers in the actual dismount fight (not including arty, attack helos, air, etc. but god's honest close combat) the aggressor has suffered exponentially greater losses than we have. Our lower casualty rate is attributable to a bunch of things - command and control, highly competent medics, training and discipline, equipment (incl. more lethal weapons and body armor), and the speed of evac.
Now look at it from the defenders perspective. He rarely knows when he is likely to get hit. Hence you wear your gear out of the gate. If he gets hit in that ESAPI He may be down, he may be out of the fight, but he'll likely live because he isn't alone. Those standoff tactics that the aggressor employs (mortars, sniping, IEDs) that have accounted for the majority of injuries have been reduced through the use of body armor. - hence wearing it based on the threat's ability and lethality.
Simplified if you don't know when you're going to get hit I'd recommend wearing it. If you're the aggressor and can fade into the public then maybe not. If you're going to play "G" against organized forces then no. And neither would I wear camo or LBE or carry anything remotely of military value that I wasn't able to ditch (probably permanently) if I had to E&E. No camo, no combat boots, no webbing, no rambo knives, no survival kits. Nothing that would say "I'm military oriented".
I hope that gave you the answer you were looking for.
