That only applies to a combat field that allows such a strategy. It will probably work in a game since the battles there are at fairly close range, which is why I advocate a mixed armament in the sections pertaining to the game. This does however not contradict the central point, that spinal weapons are more powerful as long as range can be kept.
Which is quite simplisitc. If they are worried about that, they can simply built all of their projectiles with explosives in the tip that remove most of the momentum, or shred the projectile small enough that the debris is unlikely to damage anything with an atmosphere.
That is if they don't build the projectiles in a way that they degrade after a few hours (long enough to be irrelevant in battle), for example, losing cohesion an turning into gas, which also removes some forward momentum and expands the remaining mass too much to cause any damage.
These are just some ways around this, besides the fact it is quite unlikely for the projectile to hit anything worthwhile making it a marginal concern to begin with. The most likely targets are, after all, gas giants and suns, having the largest surface. Besides, with that reasoning, using any sort or projectile weapons would be immoral, as every projectile regardless of speed continues indefinitely in space until some force stops it, and any projectile used as a anti-vehicle weapon would damage space stations or live-bearing planets hit.
Also, there is no reason directed energy weapons should not be able to be built with a higher range, since assuming a constant loss of power means a higher initial power equals a greater range, which is certainly more complicated in practice, but there is no insurmountable limit to energy weapon ranges, just an envelope of decreasing efficiency. An energy weapon that does not need to share its space with the mountings of a turret can also be built to achieve a larger range with the same stopping power.
There is nothing stopping anybody from installing several spinal weapons, however, and those could have the same mass as a turret - which wastes quite a bit of its mass on gears, conduits and additional armor, as well as perhaps independent targeting systems. Spinal armaments don't have to be very large, after all, and they have a better mass-power efficiency.
There are other advantages, such as better protection since only the nozzle needs to be outside of the hull, better power efficiency since the weapon is closer to the reactor, meaning lower losses to transmission, fewer movable parts meaning lower wear and tear, as well as additional savings in system redundancy. You only need one FCS, for example, which doesn't need to be connected to all the guns either, since their angle and orientation is always identical to the angle and orientation of the ship. I can't say that I know how much of a difference that makes at distances relevant to space combat, but it is an example. Having backup FCSs still makes sense, but depending on the necessary computing power those may be smaller than an FCS having to handle a multitude of turrets.
Meanwhile, turrets have a limited traverse, regardless of the shape of a ship, so there will always - as was mentioned before - some weapons that are not in use, unless the ship is encircled from all sides, which isn't really a good situation in combat. Most of the time, larger ships - the kind that would need turrets to keep the weapons trained on an enemy in the sort of close combat that happens in games - will not aim to enter the enemy formation, and a ship that is built to enter an enemy formation would need to have next to no weak spots or blind angles, which means it will waste a lot of mass that would not be required if it just stayed one step further away from the enemy. There is no law that says all ships need to engage in melee, after all.
I did, however, repeatedly outline the limitations of spinal weapons, that they do not operate well in extreme close combat environments, to name the most important one in games. This only applies if the ship can not turn in time to face the enemy, so it really is only a concern in games - by all accounts, detecting anything in space at a far distance is so easy that turning could not possibly be an issue, and targeting something that approaches in a direct line or comes closer than a certain threshold should be very easy, as spaceships can only dodge in a fairly limited fashion. They can also turn freely without giving up anything besides acceleration time, since their velocity is kept unless they decelerate actively.
The only real disadvantage may be that the heat-producing components are also deeper inside the ship, and heat managment is a highly critical issue in space, though most science-fiction doesn't discuss this, and all turrets carrying radiator panels also introduces some vulnerability and potential issues, like sensor interference.
Practically speaking, ships should always have backup weapons, but due to the weight and space savings, mid-sized ships with large-sized spinal weapons would work in almost any situation, including being bait for the enemy to do this, again:
The enemy has to get there first, meaning they would be exposed to any other weapons while trying this. A force of equal size would take some damage from approaching under these conditions, but nothing is stopping a fleet that is built around mid-long range spinal weapons - which is not a built I'm even advocating, mind you - from also including a few ships that are focused on close combat to the exclusion of everything else, meaning they would be superior to a less specialized ship, or balanced if the approaching enemy is composed of all-rounders while the defending fleet is built from two sets of specialists.
Even better, the spinal weapon ships can simply retreat once the enemy approaches engagement range, covering the melee ships with their own turret armament until the enemy can not evade them any more, at which point the enemy risks longer-ranged fire again unless they pursue, risking a flanking maneuver from the enemy melee ships. Friendly fire is not a large concern since the melee ships can simply retreat once the spinal ships have reached a safe distance, again forcing the enemy to pursue unless they want to risk damage from the other group. Even splitting up would only open the attacker to any number of other countermeasures, since they are certainly at a disadvantage unless they approach the longer-ranged enemy ships, meaning the only low-risk countermeasure is also utilizing long-range combat specialists, and all other countermeasures restrict the enemy to approaching first, making their behavior predictable.
In any case, a somewhat balanced approach for both the individual armament of ships as well as the the fleet composition works best, but outside of a situation where combat happens exclusively at melee range there is no compelling reason not to use spinal-mounted or fixed weaponry along with turrets, especially since spinal mounts involve very little tradeoff against turrets - how many turrets are mounted on the nose of a ship, after all? The only real tradeoff is in energy and mass available, and fixed armaments are lighter and potentially more energy-efficient than turrets.
Indeed, there may not be any tradeoff whatsoever if a ship carries a heavy turret armament, since there may not be any spots with good coverage left to place turrets. Finally, your initial point was that spinal weapons as such serve no purpose, while mine was that they do - I see very little sense in reducing player choice in this regard, and I was never arguing that turrets serve no purpose in games such as this.
There is not much point in a pure turret-based lineup due to the statistical disadvantages of turreted weapons outlined above, and the same goes for exclusively fixed-armament fleets, since both are just two types of weapons mounts with complementary strengths. There are also flexible mounts which have a limited traverse, such as on old tank destroyers, which don't show up in this game, or missiles that aren't restricted by firing arcs to the same extent as they can reorient after launch.