As pointed before, the Ion cannon is not to be shot each turn, just to set the next ones.
It also has a psychological effect. Your opponent will be very careful about his actions and moves while in your line of sight. If you happen to get him while stressed, you ionize him so next turn he move forward 1 and can't get an action. Ideally, you k-turned behind him for a good shot using your normal attack. Good chance that his next maneuver will be a green, or just ionize him again, so in effect, your opponent has been predictable for two turn in a row, one of those he didn't had any action to help himself.
So, morale of the story, you don't k-turn in front of a Defender with a Ion Cannon.