Basically, if there's an actual drop rate, use it. It only affects cases where there's multiple locations with the same item (not multiple enemies), and it's trying to decide which of them is best.
Edit: To clarify - Setting the unspaded value that low doesn't make has_goal skip the location as a possible place. It just depreciates it if there is another spaded area with better known drops. Setting the value to 5% doesn't make sense when it's possible that it's an ultrarare 1% drop or something similar. If we know that another area is 5%, use that area first.
Unspaded means that you might get the item, but it might be pickpocket only, or it might be conditional, or... we have no clue if it's even possible for you, specifically, to get. Maybe you need to finish part of another quest... maybe you need to be a different class... who knows?!
If we have a different location where we know you'll have success... go there! You'll get it eventually. The unspaded bits? Meh. That's why they're unspaded...