I think it's necessity and courtesy mixed half and half.... you can't really force, or expect anyone to stick to it strictly... but still the more there is the better for everyone.
And ofcourse.. sometimes its plain silly.
Often, I'd be dragging an enemy behind me, flying straight and level, pretending that I didn't see him... and observe how friendlies react.
While most people would give check6 calls, there are some people who's like 400 yards behind the enemy con and still would not give out a check six call.
He's using me as his bait - obviously doesn't care if I survive or not. Empirically, about 20% of the friendlies around are like this. Can't blame him for anything, but he sure won't gain any respect as a fellow AH pilot.