In order to even phrase a question, you need to know something about a domain. The words "I don't know" might be a convenient shorthand to express to the unenlightened that you don't know very much in particular—that you don't think you can do better than they can, or than your ignorance prior. But in real life, you are constantly making decisions under uncertainty: the null plan is still a plan, refusing to choose is itself a choice, and by your choices, you implicitly take bets at some odds, whether or not you explicitly conceive of yourself as doing so.