Darth Simon, have you ever played the game Homeworld? During the actual battles we get a real 3-dimensional mix of ships exchanging fire - true. This is how the battle is handled. But during cinematics opponents don't approach each other "upside down or whatever direction they see fit". Why is it handled in such "old fashioned way"? Because we need something we can relate to, something that is easily understood by our brain, something that easily speaks to our imagination. Average viewer gets confused by real 3D battles, it requires decent spatial orientation etc. I'm a flightsim fan, do you know how many people feel "lost" in air dogfights? You need some training to easily orient yourself in 3D space.

And this "unmentioned rule" stuff wasn't invented by me, I heard it in some SW documentary where people with far more knowledge about film making than you or me were describing their "tricks of the trade" used in Star Wars FX etc.