There is a lot of scientific research that shows that a balanced diet is all that is required to provide all the vitamins the body requires and that supplements at best are a waste of money and at worst in large doses can cause harm.
The only exception would be for people that have specific conditions that a doctor suggests supplements for.
Yes, but it's actually easier than people think. You don't have to make a science of it or measure doses, just ditch flavor enhancers and avoid preservatives, read the labels on stuff you buy (yes, time-consuming at first, but you'll have to do it less and less as you gain experience), favor fresh produce over junk food and get lots of sunshine.
Well, what I'm saying is that the practice is easy once you muster the will. Sadly, many people realize how easy it is after a disease forces them to change their behavior.
Plus you simply don't know if e.g. the oats in that tin of oatmeal you're working through were grown in soil that had adequate (for you) amounts of selenium.
Taking a "One A Day" vitamin just to make sure you hit all the bases is a minimal reasonable precaution.
Its a no-brainer to me that if you are worried enough about your health to simply change your diet to add a bit more fruit and veg rather than spend money on magic pills without any medical or scientific basis.
The only exception would be for people that have specific conditions that a doctor suggests supplements for.
Edit: I though i would add one example i read recently about Omega 3 that really stands out: http://www.newscientist.com/article/mg20627601.400-omega3-fi...