[Suicide Junkie]

Uhm, I thought I spelled the main benefit out very clearly...
(Lots of people quoted the post in its entirety too)

One missile might (13% chance) get through a storm of 4 PD. One PD gun might clear a pack of three missiles (1.5% chance).

With stock PD, it (almost) always kills the first N missiles in a pack, where N is proportional to the number of PD guns. All the rest get through... no luck or chance involved.