I would say that everyone would be deficient is some vitamins or minerals, there is no way we could get everything we need from food, we will get enough to live, but we as weight trainers are asking a LOT from our bodies, and have a higher requirement then sedate people.
I am not saying you are going to die without it, but after putting in the hard yards at the Gym I want to get the most out of my training sessions by giving my body the best chance to recover and grow.
There is no way we could get enough fish oil, calcium, protein, anti oxidants etc etc etc just from food we would have to consume huge amounts of all foods to get everything we are supposed to have.