AlphaGo and the Future of Computer Games: Conversation at U. of Alberta [video]
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Thus AlphaGo plays as a human (before MCTS), it's choices model a challenging oppenent.
If the dataset included the name of the player in the Dataset then AlphaGo could play in the style of that player.
This offers a human experience, and for teaching one can show what different experts at differing levels would choose.
If computer games had awesome AI trying to beat you, you'd need the overwhelming superiority of numbers just to stay alive.
That applies to the computer competing with you, though. There are games, like eg RPGs, with more space for cooperative AI.
At the time it seemed novel, and even rather hilarious. Now it seems somewhat standard (aimbots can win every time, for example).