Though the system is technically set to be a representation of the multi-dimensional facets of space, hence the ships not blocking line of sight, it wouldn't be practical to attempt to represent these dimensions; such as elevation. Plus the system as it is feeds into the standard naval terminology, and though deck guns can often rotate to fill a number of arcs they can't fire in multiple arcs at the same time.
For example lets look at the Nova's multitude of pulse cannon. In all likely hood they are mounted on rotating turrets, but rather than have the potential of 180 degree arcs the weapons fire is broken down into the arcs they are because each gun can only fire in one direction at a time. Besides any ship you want to shoot will be in one of the predesignated arcs.