Pallington No weapon has near a scanner's range so you'll always see your enemy before any actual fire can be exchanged. And they'll see you in more than enough time to face you as well. Scanners do nothing to change this. Since you see blips anywhere in the system all that matters is getting full vision before the fighting starts.
Since the scanner provides no combat advantage any ship with a scanner is strictly inferior to the same ship with a 3x3 block of corridors in its place.
It's also very inconvenient to place in most designs and eats a lot of power and manpower in the process of doing nothing constructive.