The difference between an answered and unanswered question is absolutely not the same. Nil means the user has not told us how much luggage they have. 0 means the user has 0 luggage. We can assume a lack of response means zero, or we can mark the field as required, in which case the UI needs to prompt the user for a response in the nil case but not the 0 case. This is an extremely simple, common example. There are many cases where nil is not equivalent to 0 semantically or logically.
