1. Spacecraft design has evolved since fixed mounts was first introduced.
2. It's not well thought out, since if you can do it for turreted weapons, and spinal mounts, how about those inbetween?
3. Come to think of it, wouldn't spinal mount targetting be dependent on the pilot rather than the gunner, since neither accelerated particles nor mesons tend to curve in space, unless the target is behind a black hole?