One of the biggest problems with AIL is the fact that PVP gear is usually higher iLevel, easier to obtain, and mostly worthless in PVE. PvP Gear will never be as powerful as equivalent raid equipment. This is because PvP Gear sacrifices a large amount of stats to bolster stamina and resilience.
I'm sure many of you have joined a raid with a 359 AIL player, only to find out that he is loaded up in PvP Gear and doing less damage than the tank. Isn't this infuriating? At the very least you've probably run a random heroic with a player who snuck in wearing PvP equipment.
This is just one more advantage GearScore has over AIL. GearScore drops drastically when equipping PvP Gear, to emulate the drop in performance the equipment provides. In addition, we have a separate PvP GearScore value that allows you to see how your gear holds up in PvP play. Just check out the screenshot below.
Even more astounding however, is the fact that this player was missing 5 talents! That's something AIL will never catch. Do you really want to take players to your raid who are missing talents? So stop using AIL and pick up PlayerScore already!