Yeah except you might be deficient in some vitamin, and in that case you're screwing yourself over by not taking a multivitamin that has just 100% of the RDA's. Not everyone can afford to eat a world-class, healthy diet, and obtain all nutrients sans a multivitamin.
It is medically very difficult to be vitamin deficient. Modern food is fortified in so many ways that its now almost impossible unless you do something like eat exclusively exactly 1 food product. Even then.
It's not about a healthy diet - it's about the fact that vitamins are trace components that your body holds onto what it needs and discards the rest. This is very different to the general nutritional needs of the body (carbohydrates and the like - all the things the Soylent maker is principally concerned with).
Multivitamin pills are an expensive "worried well" type supplement. Very very few people need them. They're not "generally a good idea", and if you can afford them you should be using that money to buy better quality foodstuffs because they certainly won't surrogate for poor nutrition in the major groups that you do need in large quantity.
People do still get scurvy simply by mostly eating cooked foods. It's still rare, but not that uncommon among collage students.
The sad thing is while vitamin deficiency is actually fairly common it's often a slow process and the body can cope fairly well so it's less noticeable.
By common I mean it's something that your average General practitioner doctor will encounter. Pregnant and Nursing mothers are often told to take supplements with good reason. Also of note absorption issues are just as important as diet which is one of the reason B12 shots for example are used to treat deficiency.
With that said, taking a daily multivitamin is often overkill taking it weekly is often just as useful. It's just that they are cheap enough that trying to figure out the ideal dose is generally a waste of time.
"By common I mean it's something that your average General practitioner doctor will encounter"
Well, sure, but that's during specific cases where the person knows that there's something wrong with them. It's not commonly "accidentally" discovered during the course of your regular physicals.
Women who want to become pregnant, or who are already pregnant, should be taking 400 micrograms of folic acid (from before conception to at least 12 weeks conception) and 10 micrograms vitamin d, but avoiding anything with vitamin a.
