Enemy advice from President Ark on Somethingawful forums:
Regarding the tainted NPC drops: Don't sweat them, nothing they drop is irreplaceable. Most of the ones you run into commonly just drop some basic thaumcraft stuff and/or cosmetic armor.
The only ones with drops I'd consider notable are:
Tainted Sniper: Found mostly in the floating city, drop the components for the iron longbows they're using which translate into 5+ iron ingots per kill.
Tainted hero: One-of-a-kind in the floating city, drops some heavily-enchanted armor made from high-end materials like altarus, sanguinite, etc. It'll be nearly broken but if you can repair it it's pretty cool gear.
Tainted king: One of a kind in the floating city, drops a cool crown and all the components for a silverwood wand, albiet with inert caps so you still have to do some infusion