The received wisdom here is that if an aircraft is capable of returning from a mission with heavy damage in a particular area, that area probably has enough str

The received wisdom here is that if an aircraft is capable of returning from a mission with heavy damage in a particular area, that area probably has enough strength/redundant structure already, and adding armour isn't going to accomplish much beyond adding weight. If, however, aircraft rarely or never return from a mission with significant damage to a particular area, it's probably a good indication that damage to that area is not survivable (which is why the aircraft are not returning). One needs to pay attention to the data that aren't present, or at least wonder why they aren't present.