I have tried dedicated pd ships to little avail, the way PD (at least ballistic PD) works mean that intercepting missiles coming for someone else is extremely difficult, range notwithstanding.
As an experiment, I doubled the range and proj speed of all PD weapons and halved their damage. I then halved projectile hitpoints to make PD weapons just as effective at intercepting. This makes PD escort ships (especially 5 flak enforcer) a lot more viable but may make missiles underpowered as a result. More testing is definitely required.
Side note, looking for some ideas here: How effective should missile weapons be, do you guys think? Say a lasher launches 2 harpoons at another lasher, where both ships have reasonable PD capability (say 2 vulcans) and enough flux to fire them. Should any of those missiles impact? What if the target only has one PD?
What if it's an enforcer's 4 harpoons vs another enforcer's 2x flak? Does that change the equation? A venture's 8 harpoons vs an enforcer with 2x dual flak?