For the sake of (friendly) arguement:
If, on the other hand, you are of the school of thought that the ships are actually much farther apart than is shown on screen (it being more exciting to actually see all of the ships firing on each other), then your argument doesn't hold up. Even the more cumbersome ships are shown being able to turn 180 degrees in under thirty seconds.
These two sentences undermine your own arguement. You're stating that on the one hand, the show is inaccurate in the engagement distances of ships but on the other hand it's accurate in the time it takes for ships to manuever. It is entirely plausible that the manuevering time is compressed for cinematic benefit, just as are the ranges. Either the show is accurate and canon or it's not. No cherry picking. ;-)
If the target is two or three hundred miles away, a spinal mounted weapon could be brought to bear in a matter of seconds (fractions of a second if the ship is pointing in approximately the correct direction).
For the sake of arguement, let's say ships are fighting each other from miles away. At that distance, minor angles become major misses. I'm not a math person, but if an enemy is 1 degree below the omega at 100 miles away, the laser is going to miss by a huge amount. (somebody can figure this out if they want)
From an energy POV, which is easier: moving the entire ship or moving one laser cannon? Why would you expend that much energy to manuever a 45 million ton ship.
Secondly, even if we ignore the previous point, what happens when you face multiple enemies? In order to line up the boresight beam, manuevering the ship will bring your other weapons out of line with the enemies.
Simply put, it is much smarter and more energy efficient to mount just about every weapon so that it can pivot.