Different reasons for why the principle doesn't hold water resonate with different people. The tax issue was more about "if you already do this, beware of ...", not about why it's wrong (that's the rest of the post).
eevilspock, who is obsessive about food, the anorexic saying "I don't need to focus on food" or the person saying "there are different ways of utilizing food, I should be thoughtful which is healthier for me and for my current goals"?
There's a case to be made that being the third person who says "All my life are defined by the exact amount of calories I'm eating" is unhealthy like the first one, but saying that these are the only kind of people who gain value by food advice is a stretch.
When the iPhone App Store had just launched years ago, I saw that all of the tip calculators focused on exact calculations, even making a point of excluding the state tax from the tip. At least one went so far as to incorporate a state tax rate database! My idea was to write a tip calculator app that put things in perspective, that rounded and swagged, with the intention of making tipping and payment easy while influencing people's perspective and generosity (e.g. Just put down a $20 bill rather than $19.42 or if you're paying CC, tip $4 in cash rather than $3.91 on your card). Then I realized that by having people use a tip calculator I was undermining my own message! Anyone who is uncomfortable with "Just take the bill, divide by 10, then double, and round. If you aren't poor, be generous" just doesn't get it.
What is the cost of living an anal, money-focused life? See my other comments.