I tried a simple test on Stage 31.
For the first run, I used only Troopers with no Missile Turret. Then I played the same stage again with the same basic units, but added a Missile Turret to the defense.
That one change made a surprisingly noticeable difference. The setup with the Missile Turret managed to hold out almost a full minute longer.
It's a simple comparison, but it shows how much one extra defense can change a battle in Hero Wars: Zombie Virus.