It only takes a couple to make a Broadsword vulnerable to Vulcans, though. And they're really quite nice AA on a ship with no PD slots. Like, say, a Pirate Wolf.
If you're spending 4 or 8 or 16 OP of valuable missile slots in order to slightly improve your PD/anti-fighter, you'd be better off considering IPDAI or even just more/better PD/AA and getting proper missiles (you even save OP if you opt for Reapers or Hammers!).
They aren't a replacement for a lack of AA options. If a Wolf (P) has two Swarmers, it takes at least 7 seconds for it to kill a single Broadsword, by which point they've already dumped their flux into your ship. And then you remember there are three of them and you have to hope that the targeting AI actually hits the same one. And then you remember they're probably coming from a dedicated carrier, which means another wing of something, and that even if you lived long enough to use all your Swarmers you haven't even made a dent to their replacement rate. Better to just focus on whatever a Wolf (P) is actually good at than to try and fail to make it usable for a role it can't do.
I use Broadswords as an example because they're tanky enough that you might think you need the extra firepower of anti-fighter missiles to take them out. But Swarmers aren't helpful for them, and regular PD/AA does well enough against more fragile fighters. 13 hits isn't a few. It means you have to use like 40-45 ammo to take down only the initial wing. You can say they supplement PD/AA, but I don't think there are any ships that need it that can't also simply fit more PD/AA or are a lost cause altogether (like the aforementioned Wolf (P)).
They simply aren't economically worthwhile.