How Beam Dissipation Works

[vmxa1]

Not only does it waste them (PD), they can only shoot a short distance. When you select any PD mounts from the starbase or whatever orbital you have, it cannot hit missile that are a long ways off.

[Brutalisk]

And it's not reccommended to try and use them even when they can reach unless they won't get a chance to (auto-)fire otherwise...

(they auto-fire when missiles are about to hit that specific ship or starbase)