IMO they should never be used this way period, or at least not without also indicating the values with and without the change. You'll be accused of being dishon

IMO they should never be used this way period, or at least not without also indicating the values with and without the change. You'll be accused of being dishonest if you use a number that is convenient to your argument (e.g. "buying this is good value!") but which a significant number of people will interpret incorrectly.