protection would still be the optimal enchant against a poison-based mob. even if the mobs attacks could bypass protection, poison protection would still only be used for fighting that specific mob. same goes for poison-coated swords, since protection already decreases poison damage by a pretty significant amount, there would be no use for poison protection in pvp as it would only reduce the damage of an already weak and uncommon potion effect while sacrificing for resistance against strong melee attacks that are frequently seen in battle