Originally posted by Nobbler:

You don't have to go into Mistlands. If you are a completionist, then complete ML and stay out. Don't use ML gear if it is so bad.

This makes no sense considering you need to go into mistlands to most likely progress after the biome. So no it isn't a "completionist" thing, it's directly in the progression, unless it is completely optional within the fully complete game, then it would count as completionist content, however it most likely isn't. This logic will fall right into the ground once the next biome releases and you need to go into mistlands to progress further.