I wondered about this as well. I am not sure why they have it, they certainly didn’t have it when I played on PVP. I first zoned into battlegrounds with appearance armor to try and hide what class I was and yes I often play as my healer. Most players especially the good ones, will work out what class you are fairly quickly, but with the symbols displayed over head I think it over simplifies it. I got rid of my appearance armor and weapons as soon as I realized that it was pointless. Now I have to hope that I get grouped with a good tank.
I would prefer if the symbols were removed or maybe you could see them only with-in your group/raid.