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What Sandy said.
Certainly there is some luck involved, but WPA is like RBI. You don't get 147 RBI without being very good, and you don't get 147 RBI if you are a 'soft' player who gets a lot of your production after games are already decided.
If I told you that Mike Carp was going to have 158 RBI next year, you could complain about luck imbedded in the stat, but you would also know that Mike Carp was going to be very good, and that he was going to hit when it mattered.