It's worth noting that they're describing the effects of high doses of vitamins. Given the average modern diet (at least, for Americans like me), vitamins may be necessary to achieve "normal" doses.

I take a fiber supplement (which, I know, is not a vitamin) because I try to avoid refined carbohydrates, which tends to mean I have to avoid carbohydrates altogether except when making a home-cooked meal. Since the average adult male around my age/weight needs ~28g of fiber a day, and I probably consume 15g or less, the 6g supplement seems prudent. Anecdotally, my ... er, system ... seems to work better that way.