It would be nice to have the question ID included in the response JSON to track if a user already answered a question and if he answered it correctly. Also, a way to request a question via ID could be helpful in some use-cases, e.g. when building a multiplayer app and you want to provide both players with the same question.
I agree, having the id would be very useful.
I've spotted a few formatting issues with a couple of questions that I'd need to correct prior to consuming in my app, so having a known list of IDs is much faster to process than string checks.