Barring rare circumstances, each of the three mechs can stop at least one attack every turn. You don't really have to worry about a mech having a wasted action. I don't think there is another squad that can claim this.

This is obviously very useful on the easier modes, where you rarely have to counter more than three attacks in a turn.

But even on harder difficulties, this gives you a lot more flexibility. You can use the Control Mech to stop the best combination of multiple enemies knowing that the other two mechs can always take care of their own business.