The Galaxy class has several secondary phasers dotted around the ship, but they are almost never used. When the ship is attacked from behind, why doesn't it fire the aft phasers?
When shooting The Next Generation, Effects shots were different than passing shots of the space ship. According to producers they did about two dozen or so passing shots of various angles of the ship moving thru bluescreen (space) and used them over and over throughout the entire series. albeit with different backgrounds of planets, starfields, etc. So the ship fired from it's prominent front end to showcase the visual effects shot. Since the front is the most prominent scene, firing there makes the most amount of sense. It's a "Beauty shot" As the series progress with CGI, new effects shots in the fourth season and beyond allowed them to do things that were too expensive to reshoot with models. The USS Enterprise-D fires at the Tamarian vessel in TNG episode "Darmok". The phaser beam famously emanates from the torpedo launcher. A mistake later fixed in the reformat to HD. There are a few instances where it fires without it's saucer section firing. The episode "Descent" it does fire it's underbelly phaser on the stardrive section. Another instance of firing was in the Season 2 Episode "Arsenal of Freedom" where it detaches the saucer section and fires from the "Cobra head" of it's neck section. In the episode "Best of Both Worlds" the ship fires it's nacelle phasers, a rare instance. Plus I don't know what "Secondary" implies the saucer has the two longest running sections, but they're by no means presumably more powerful? The ships phaser arrays are built throughout to cover any potential blindspots so it can fire in any direction.
Honestly I always thought the same thing, but in my opinion when the ship is attacked from behind, it seemed like the preferred method was using the aft torpedo tubes to fire back. As to the reason why the other phaser arrays weren't used, there are two large arrays that encircle the dorsal and ventral surfaces of the command module that were larger then the other 9 and because they were bigger they were always preferred. The other 9 smaller ones on the engineering section were used when the Saucer separated, which is why the engineering section on galaxy class ships were sometimes called the battle section because it holds most of the ships weapons. Hope it helped!
There are a number of smaller arrays along ships contemporary to the Galaxy-class and Sovereign-class starships. Note, however, that these are "arrays". Multiple energy generators (I do not know the canonical term) build up the energy along the array, at opposite sides of the array. The two points of energy buildup come together, the point of which is the origin of the phaser beam. If we can deduce that the more energy generators, the longer the array (and vice versa), then the most powerful arrays would be the primary ones on the top and bottom of the primary hull (aka saucer section). So it makes sense that it would use its most powerful array unless 1) The ship was for some reason unable to position itself to fire from its primary array, or 2) The ship is engaging multiple vessels.
As to the second point, I'd have to go through DS9 again but I'm almost positive that some array-bearing starships were shown utilizing their aft arrays in at least one of the big battles.