I was going to say the same as Lutrus. You said defending was pointless. He showed its not pointless and the reason you defend is to gain that defense point.
If your team ran waves (multiple attackers on different targets at the same time) then they couldn’t defend them all and would miss out on points.(They would have less than 50 defense points) The Merc backs them one at a time so you can get a point for every run. In this way you get more defense points and win a 250-250 war. Simple tactic that negates the usefulness of the Merc.