You don't need to know all the gear one player has. Just the main pieces gives you an overview of how resistant he will be or not and how much DPS he'll deliver.
The amount of gear he has, and where it came from gives you an idea of how much farming he's likely to have farm and thus guess the amount of AA he's likely to have.
Pair that to guild he's in and PvP level and you've got a rather accurate picture of what we's wearing.