Obviously, all vitamins are important to a healthy life. Vitamin D plays a critical role in a wide range of bodily functions, from mediation /support of the immune system to maintenance of the skeletal system. The Recommended Daily Intake required for healthy functioning is miniscule - micro-grammes per day - so it is difficult to be deficient in healthy quantities. We can also synthesise more than the RDI with just a few minutes exposure to the Sun.Taking more than the recommended daily intake is unlikely to harm, but it is unlikely to do much good either, since extra quantities of the vitamin are just excreted from the body.
That having been said, of all the vitamins out there, as a resident of a western society in a largely overcast island, Vitamin D is one of the few vitamin supplements I might actually consider, especially if you have an unhealthy, bland or unvaried diet and live in particularly overcast areas of the UK , as I probably do :)