Agree with you usand on the overall diet info, but not re nutrition. Yes, horses do usually survive well enough without well balanced nutrition, as we do. After all, a lot of nutritional studies & subsequent supplementing is quite recent & we (mostly) survived without our 'vitamins' before.
But I don't think *appropriate* supplementation is a waste at all and there are many health problems associated with deficiency/imbalance. For eg. Horses can have weak feet due to nutritional problems and they appear to be far less susceptible to infections if they're getting a well balanced diet. Studies have also shown that (even assuming a well balanced block, which they often aren't) horses generally get very little out of a salt/mineral block. I'd definitely be wanting to give a youngster the best nutrition, for the best chance at a healthy life.
I just wouldn't be taking the word for it about necessity, balance or amount from the manufacturer of the supps or from nutritionists employed by feed co's. But that's another matter!