Well, it's the principle. If you don't have enough PD system, you're srewed because YOU DON'T HAVE THE RIGHT TOOL TO DEAL WITH FIGHTER. It's quite an interresting modification of the game, i think this would increase fleet variety because you would'nt just be able to put only big weaponry, or you can but you need an PD escort ship.
Lel nice contradiction, how would REQUIRING certain weapons on ships in any way improve variety. The whole point of the customization aspect of the game is that you can achieve a certain goal with different loadouts (see, having actualy options). Imo this is a terrible idea since it would make fights infuriating, you'd have ships that can't do *** vs fighters, and others that just completely eradicate them. I hate these kinds of mechanics in games since it forces you to play in a certain way unless you actually want to get ***.
You're not
required to take anything.
I can opt for literally zero PD for every one of your ships if I choose. I can opt for exclusively PD weapons for every one of my ships (Burst PD, Vulcans, Flak Cannons, The Large PD Laser), though I'm not sure why I would want to. Or I could take only short ranged weapons. Or maybe I only take ships with hard-mounted Safety Overrides.
That just means when I end up encountering some jerk missile boats with Sabots or carriers with Tridents with your PD-less fleet (or Dooms for that matter), or if I encounter literally any combat vessel with my exclusive PD fleet, or if I encounter any fast ships with long-ranged weapons in my short-ranged fleet, or if I encounter a Cruiser or Battleship fleet I can't take out fast in my SO fleet, then I'm going to have a bad time.
Removing the ability to transform my Plasma Cannon or HIL into an improvised all-purpose gun that can obliterate entire missile salvos or an Astral's air arm feels more like removing a cheesy player-only move, not reducing the player's loadouts. There'd be an additional advantage that a single Talon can't eat your Hellbore shots.
Fun fact: In WWII they did use the main and secondary guns to shoot down fighters. Granted, they used different ammunition than when firing at ships, but the idea that only the small machine guns or "AA" guns were used is a myth. In fact, the main/secondary guns were longer ranged, more accurate, and had higher kill rates.
The US's primary destroyer weapon and popular secondary gun on larger ships was the 5" 37 caliber gun - specifically designed as a compromise gun capable of shooting at high elevations to hit aircraft while still maintaining anti-ship capabilities.
There's a big difference between the earlier single-purpose guns and the dual-purpose guns. The dual-purpose guns were designed from the outset to do both roles and would have the required high elevation, turret slewing speed, AA shell fuze setters, and AA fire control systems to actually pull it off, unlike a single-purpose mount.
I'd argue against the idea that they were necessarily more effective. A majority of the AA kills by the U.S. Navy were from lighter AA guns like the 40mm and the 20mm, not the 5" or 3" dual-purpose weapons. AA ammunition for really big guns like the 16" aren't even a rounding error and are in the "other" column.
https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/a/antiaircraft-action-summary.htmlI like the general concept, with some weapons being able to attack aircraft while others that really shouldn't be able to in a 3D environment, can't. Rather than just point-defense weapons, though, I'd do an "Anti-Fighter" or "Dual-Purpose" Tag for dual-purpose weapons that allows weapons to hit fighters. It would include all the PD weapons, plus some others like the IR Pulse Laser or needlers. An argument for, say, Hephasteus Assault Guns being dual-purpose could be made, but I highly doubt you could do the same for something like a Hellbore.