TLDR; I've found a moderate investment in PD to be useful for low tech ships vs ordos.
My current thoughts from a low tech playthrough that farmed double ordos for a while without an endgame setup (level 11 main, haven't found more than 3 PCLs total, mixed S mods/officer skills as a lot was experimentation, my skirmishers were A hounds because I wanted to stick to the low tech theme, etc). I was taking the occasional lost ship recovered with a D mod, but I've got the industry skill to repair those and the number of alpha cores rolling in still made it very profitable. I was not trying triple ordos or above, as I was pretty sure I would lose. So thats the context for this:
I found moderate PD to be a useful investment because, while I don't use shield shunt, my ships do get maxed out on flux when they find themselves in a bad situation (Radiant jumping a Dominator, cluster of Apexes, etc), and with moderate PD they can stop moderate missiles (which most enemies have) from stripping their armor, letting the ships last longer.
On my onslaught I am using 2 central flaks, 4 vulcans on the rear arcs (in the medium slots) and 6 dlmgs in the front (for brawling with elite pd). I've got elite PD and S modded turret gyros (mainly for the damage boost vs other targets). With that setup, I can shoot down "normal" missiles: harpoons, pilums, torpedoes, rockets, etc. It can get most of a hurricane, sometimes all, and it can blunt a squall storm but not stop all of them. For me thats an acceptable level of PD for an acceptable investment considering that I'm using the lmgs as assault weapons too.
My wingpilot onslaught doesn't have EPD and neither do all but 1 of my 3 dominators. On the onslaught I'm using an identical PD setup and it works despite the lower range/damage (I'm thinking of swapping the dlmgs for vulcans and removing the flaks/substituting them for more offense as the vulcans take up the slack, but those slots are just SO good for flak). On the dominators (except for the EPD one) I'm using a vulcan ring as they don't have S mod turret gyros - I had the two side mounts empty for a while but I saw too many skirmishing frigates sinking missiles into the blind spots, the cheeky little buggers.
On the topic of missiles balance: the only missiles I think are currently very out of line are proximity charge launchers. Harpoon spam is more immediately deadly, but is ammo limited in long fights for everything but gryphons, and much lower total DPS (its more a fleet commitment thing). PCLs + elite missile spec on an Onslaught or Dominator is just a brutal amount of DPS, and they have plenty of ammo. Like annihilators +++.