The classic example is if you take multi-vitamins, you usually have noticeably different colored urine. That's no coincidence.
Personally the only substance I've noticed cause substantial colour changes is high doses of riboflavin (the infamous neon yellow...).
Several of the vitamin B's are often found in energy drinks and various pre-workout mixes in very high doses as well (e.g. it's not uncommon for pre-workout mixes to trigger niacin-flushes as well as riboflavin-neon color). The motivation seems to be that the potential benefits might be good enough and the risks low enough that it's better to dose high and maximize what is available to the body, even if most ends up being excreted.
Not many other supplements tends to be dosed at such high multiples of RDA's as some of the B-vitamins often does.
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.
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.
The persons who claim this are usually trying to push nutraceuticals at the same time.
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.
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.
(http://www.nhs.uk/Conditions/pregnancy-and-baby/pages/vitami...)
You'd have to also declare that it's impossible for people avoid these vitamins in everyday life before they became useless.
However, I though the general advice was to make sure you get your vitamins from your normal diet (as you'll get other benefits too).