Disclaimer: This is just my own opinion on icons in flight sims and has nothing to do with what I see for FSO in the future.
First off, the ability to glance at an airplane that is 3000 yards away (short icon range in AH) which is almost 2 miles and immediately tell thats its friend or foe is many orders of magnitude easier to do than telling if it was friend or foe in real life. Even modern fighters with 4th generation electronic kits cannot identify a/n aircraft that easily.
Secondly, and this is important to remember; in the Second World War there was much more censoring of the media than there is now. You were not permitted to write about a lot of things that happened and have then published. Stay with me here, the point is coming...papers of the day did NOT EVER, have an article about what we call now "friendly fire". Further to that, when loved ones were notified about a death, the information rarely if ever included much detail, and it would certainly never have said if Captain X died from an incident from that sort of thing. It was assumed, in almost all cases, that the departed met their end at the hands of the enemy.
So where does that leave us? it leaves us with a somewhat "slanted" view of how common "blue on blue" fire was in WW2. It was not reported to the public, pure and simple. It was im sure considered "bad for morale" and all that. Even amongst the fighting services, it was bit dwelled on for similar reasons.
The fact was that shooting at friendly airplanes was a common problem. WW2 air combat happens at speeds of 200-900 mph (depending on the angle), in hazy, cloudy, sometimes low light surroundings. Its not like walking up to a parked P-51D at an airshow and having a nice long look at it in the noon day sun from 20 feet away nice and calmly, without worrying about getting killed while flying in a real dogfight, with your adrenalin pumped through the roof.
The pilots in those days had no way of knowing who was who without getting close enough to identify an a/c with eyeballs only. Despite HUGE national insignias on the wings and fuselage, they still fired at each other sometimes, even at close range.
My point being that more often than not, enemy a/c were identified by a variety of factors: formation they were in, alt, a/c type, direction of travel, ect, as well as color schemes and finally, closer in, sillouettes and markings. There was no "red sign" sitting above the a/c telling you its an enemy. If there was there would have been no need for D-Day invasion stripes, ID stripes, national insignias and a/c recognition intel posters (which judging from some accounts more than a few pilots and AAA gunners should have spent more time looking at).
So I take the contrary view of some posters and say "dont think because you have icons that has anything to do with reality". Icons are there to help in GAME PLAY, to make it easier for the casual gamer to know who the bad guy is, just as in flight IFF radar is in the Main Arena.