Essentially it takes the control of them away from the player. You have no choice in whether they attack when in range of an opponent, the dice are rolled regardless and they either do or they don't.
So it's not so much an advantage to the player whose troops they are and it costs points to remove it. It's more like a trait which you have to take into account when you use them, i.e. holding them back out of the way until you are ready to use them hinders you. While it pays dividends when it works out in your favour, as Ray says, skilful play by your opponent can turn this against you.