PA’s are amazing, because (IIRC) 60% of their damage is Absolute; meaning they completely ignore resistances. Which in turn means, that against resistance-focued shields, it cuts through them like butter.
If we take this against my (planned) Vulture for example, with 4 pips into SYS, it’s raw shield strength is ‘only’ 3510 MJ (counting the SCB as well). However, due to resistances, it’s effective strength is anywhere between 7000 to 9000 MJ (depending on the type of attack). So every other weapon except for PA’s will have to fight through 7000-9000 MJ of shields, while 60% of the PA’s only have to fight against 3510 MJ.
Which is why, on big ships, resistances are slightly less important than on smaller ships; smaller ships are damn hard to hit with PA’s, so they have less to fear from them.