Its designed so everything is a trade off, and balanced together, if you want missiles you have to sacrifice something else, It would be missile spam if everybody was carrying them,
As i mentioned the X series in the Flight Assist Off thread and in other threads, all its' ships can fire and load missiles independently of their main guns.
Missile use is balanced in the game by them mostly being expensive (while cheap missiles are so weak, they're best used to get mission targets attention or to kill enemy missiles), always taking up cargo space, something you fire separately from your main weapons lest you destroy them with your own shots, and away from friendly fire and are largely target dependant.
Regular single missiles are great against light fighters (most of which lack rear turrets) and about half of heavy fighters (the ones which lack rear turrets), but are largely wasted against fast scout craft (which normally outrun them), or usually anything with one or more turrets.
For example, if i see an Argon Buster or Teladi Falcon at range, i think great, "Missile time!", but if i see an Argon Discover (too fast) or Nova Raider (rear turret), i think, "Damn, now i have to get in close and use my guns...) .
Regular missiles can overpower a single turreted fighter, but even then the turret while destroy probably 60% of them, so it'd be an expensive target.
Swarm missiles then are designed to overpower turreted ships, relatively cheaply (since most fire around eight warheads each) .
Overall regular missiles are well balanced, though missile frigates (and to a lesser extent bombers), introduced in Terran Conflict could devastate an entire sector, though their missiles are expensive to buy and slow and hard to find on the open market, after millions in investment and maybe a month or two (probably an exaggeration, though not by much

) playtime manufacturing them could be an "I win!" button for the player, but i'm not calling for them, just regular missiles

.
(Though again powerful in a players' hand, were pretty useless when flown by npcs in TC, 'til Albion Prelude "fixed" that particular problem, when they suddenly became what they were supposed to be, the sort of thing that the sight of seeing an enemy flying one, would make grown men run away crying! or even when they're on your side, curse them for stealing 90% of your kills

.)
Also while X has some pretty thirsty ammo based main weapons (in particular i love Mass Drivers on smaller ships*), their use is offset by letting you carry alternative/back up weapons as cargo, which you can switch over to in about two minutes in space - you have to find a suitable break in combat or risk being caught defenceless, but they mean not having to go all the way to a station to rearm or change weapons

.
(*They completely ignore shields, literally eating the hulls of smaller ships, though they make slow work of killing big ships, though ultimately, given enough ammo and time, can kill any ship in the game. - This'll make more sense to fans of the series, but i once killed a Xenon M2 with a Nova Raider in Reunion, it only took about half an hour and about three cargo loads of ammunition.
In the same game another time, i killed the a Khaak M2, kiting it with an Elephant armed with Alpha Proton Pulse cannons, pity the ship lost that ability in TC when alpha and beta versions of weapons were retired, replaced by stronger variants

.)
While i know X is a single player game, but that doesn't matter much, in terms of game design.)