In my case I am an endurance athlete, so at minimum supplementing electrolytes is helpful (also during the fast electrolyte supplements have at least seemed beneficial to my experience). Also, most American's are low on Vitamin D, and trust me I prefer it from the sun myself, but currently Im an office worker, so I supplement that.

Anecdotally, I can tell you that historically I used to get sick 2-3 times a year minimum, and since I began taking vitamins 5 years ago I got sick 1 time.

But I do generally agree with you about vitamins and minerals coming from diet over supplements, but I also say if you can get all your vitamins and minerals from food, your just not running enough (thats a joke).