While Chainmail is indeed useless, I think its value is incremented by its "rarity". Find a villager, find a blacksmith, trade until you unlock last trade, drop emeralds for the set. It is a great deal of trouble, especially for a new world. Then you never actually use it, but you keep it around for the "cool factor".
I think it's somewhat up there with the Dragon Egg. Useless, but you wouldn't mind sporting it in a frame in a trophy room. I bought a chainmail armor and put it in the "Rare items" chest.
I stand firm in saying that Poisonous Potato is the worst. I have maybe 6-7 in the "Ingredients" chest, waiting for the day it will finally see an use.