Is this because of the dex reductions related to wearing armor? I thought there was something about dexterity contributing to piercing as well, which if I'm remembering that correctly may balance out any armor dex debuffs.
I believe he was referring to the increasing presence of elemental immune mobs. The dexterity penalty from wearing armor is easily avoidable, and the piercing from dexterity is small and only subtracts from defense rather than protection so it is hardly worth noting. It would be fair to say that Ranged, Melee, and Fighter are all going to be somewhat less effective while fighting elemental immune monsters in most situations.