Lack of icons is not more realistic but less.
The reason is that, in real life, it is not hard to pick out the aircraft against the background when you are in a fight. On screen, though, it can be hard without icons even when the planes are within 1000 yards of you, depending on background and color of the plane. Hence icons make it so that it is as easy as in real life.
The only question is at what distance should there be an icon to make it realistic, not whether or not there should be an icon.