Point defense ships are only really useful if you're fighting someone who uses buttloads of fighters/missiles, which so far, the AI doesn't, ergo you're only likely to see a benefit from them if you're fighting a human opponent, which unfortunately means that your battles will be fought by the AI, who really doesn't use them to their fullest, so you're better off just arming your main battle ships with PDCs, considering 2-3 of them can rip through an unholy number of fighters/missiles.
And yes, I made my sentence that long on purpose.
