Maybe the reason Garrus spends so much time calibrating the Normandy's main gun is because they didn't get a single shot off when the Collectors ambushed them at the beginning of ME2.
As long as he's aboard the Thanix cannon will run 120%, fully online, ready at a moments notice. As long as he's here. Shepard will never be blindsided like that again.
They failed her one, but as long as he's aboard he never will.











