So... I could write a big ol' paragraph explaining the first reason, but I think you can pretty much see it for yourself. Check out how my AIL on my warrior alt stays at 276, despite the fact I'm nearly naked! Where as my GearScore is scored based on the total of my equipment. (My one weapon)

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List of Reasons GearScore is Superior to AIL.

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• GearScore only counts the gear you're actually wearing.
• GearScore includes gems & enchants in its calculations.
• GearScore gives large penalties to off spec equipment.
• AIL gives equal value to PvP Equipment!