The only possible downside to this is that any data gathered this way would be biased for a PvP scenario(i.e. equal number of equally powerful enemies), rather than anything that could happen in PvE.
True, although after consistently backstabbing 2 to 3 enemies to death on the very first turn with just one lv 4 elven rogue I have decided that the current AI is not much of a challenge in the campaign and certain things need some re-balance (ahem flesh sacrifice & warlord). Testing different builds against human opponents should prove to be more interesting.
Also this can be a good opportunity for Larian to tweak their AI. Then they can make some encounters in the campaign where players run into hostile 4-men min-max parties using dirty strategies originally made by other players and now controlled by AI on steroids.
