Larian probably included Underdark creatures because of Drow. It's unique, to say the least. I'm not saying I agree with their choices, but I figure that's the reasoning. You might make a Drow Druid, so gotta give some Underdark Druid choices. Right?
I'm starting to think more and more from an options standpoint. It is just an option. If it doesn't make sense for your character, don't use it. Pretend it isn't there. I think that's what Larian is doing. Provide the option and let the player decide what they want to use based on what fits their character.
