>>35413303Cipher's ultimate goal in Colosseum was just to attract rich people to Realgam Tower to fund their world domination plans.
In-universe, they were just using Shadow Pokemon as a gimmick to get rich people to pay to watch battles in their colosseum. So it's quite fitting that they have no real practical use in battle, they're just more aggressive and violent than regular Pokemon to make the "shows" more exciting.