Blood tests are incredibly expensive in comparison to vitamin pills. Creating particular mixtures for each person would be incredibly time-consuming; you'd be better off just buying a number of different pills with different vitamins or minerals in them...but even that would probably be more expensive than just bundling all the vitamins and minerals into one pill.

Also, a person's intake of vitamins and minerals could change over the course of a year, so a yearly test wouldn't be too informative. By taking a multivitamin every day, a person assures that he is constantly getting a minimal amount of each vitamin and mineral. I say that we should just take a half-dose of each vitamin and mineral, and take larger doses of specific vitamins and minerals if our doctors recomend it.

Vitamins are meant to supplement diet. The body will store some and pass others in excess, so the filtration system in effect does the vitamin customizing automatically. Overdosing of vitamins over long periods of time can tax organs in the body in a similar manner like substance abusers. Taking vitamin supplements daily is probably not needed by most people in our society.