lol I mean, sure, this is an in-character, lore-based rationale – goblins like money, therefore they want to make you spend money (or at least just give it to them) – but it falls apart because the way one would acquire these items would be to buy them in the auction house.
Guess what else Undermine doesn’t have? (I’ll give you a hint, it rhymes with “groction blouse”)
Personally, I don’t like the idea that I should have to give up a bag slot to make up for the fact that a major city/questing zone does not have an NPC who is supposed to provide a service essential to my class. If I don’t have said item to be able to access the stable in the field, that’s fine, that’s on me. But when I am at an Inn, or in an area designated as a town with NPC vendors and services available to the player, and there’s no stable master, that’s not something that my cash and my bag space should be making accomodations for.