Try it and prove them wrong is all I can say. It does not surprise me because most Vets are totally clueless when it comes to nutrition and how it applies to horses IRL.
I have been doing it for about 40 years now. Both it and the Calcium Deficiency problems were called to my attention by an old Nutritionist I ran into about 40 years ago. I contacted him about a set of calves I had in late winter, that lice were just eating up. I had contacted the local Vet Clinic and been there 3 or 4 times and bought everything they had. They were on feed, but at that time you got mixed, ground grains or rolled grains with molasses in them and not much else was available. They had quit growing and gaining weight and I had used Malathion and Seven Dust on them and still could not get rid of the bad hair and lice. Then, my horses started rubbing their manes and I found lice on them, too. I was beside myself. If that was not bad enough, the one living outside had fungus scabs all over them, too.
The local Vet had me dusting and doctoring them about once a week, which for the calves had me putting them through a chute and really stressing them. I was afraid they would end up with pneumonia. Even though these old Vets were clueless, they did tell me that the calves would get better on their as soon as the grass greened up. [Gee -- that was when they would get Vitamin A from the grass. What a thought.]
At that time, I was giving lessons to three children whose family had a big dairy about 20 miles away. I was visiting them and getting set up for lessons as soon as the spring thaw was over. I was complaining about my calves and horses with lice and fungus. Bob, the dairy owner said why didn't I just 'fix' their nutrition problems? He said that was all it was. That he used to fight lice every winter until he found the Moorman's salesman and the Moorman nutritionist straightened them out. So, I got the name and phone number of his nutritionist. He was on staff at the Moorman Feed and Mineral Company.
At that time (about 1970) Moorman had nutritionists and Vets on staff that worked with feedlots, dairy farms, hog and poultry operations, etc and they tackled all of their health problems through nutrition. They were decades ahead of their time. Actually, Moorman was the first company to add Vitamins and minerals to livestock diets to correct problems. Mr. Moorman started the Company in about 1880 and started bagging minerals in 1926. ADM bought them out in about 1980 or sometime around then. Here is a link that tells some of the inovations they worked out in animal nutrition. http://www.admani.com/horse/Horse%20...%20History.htm
Anyway, this Nutritionist (I can't remember his name for the life of me) came out and looked at my horses, looked at what I was feeding them and told me that all of my problems were nutritional. [They were also eating trees and chewing wood -- sound familiar?] He told me that they were mostly deficient in Vitamin A and Calcium. Explained that they needed that and Magnesium, Zinc and Vitamins D and E because of the symbiotic relationship of them. If any one was missing, none of the others was available. He also said that the Vitamin A problem was responsible for the goopy and runny or crusty eyes, bad feet, scruffy skin and a lot more. He said I would never have to 'clean' another mare if I started supplementing Vitamin A (and I have not had one retained placenta in the last 40 years except in horses that had just arrived to foal out.) He said they would breed back better (and they did) and he said that supplementing Calcium and the other minerals would prevent almost all calves or foals being born with weak or crooked legs (and it did). That old man had forgotten more than any Vet I have ever run into when it came to nutrition. He is also the one that told me to use the injectable A. He said to inject it and it I had any big swellings, I could always use it orally. I gave probably 30 or 40 shots of it before I got a HUGE swelling and a horse that had such a sore neck he could not reach the ground with his mouth. So, after that, I have just used it orally.
At that time, Moorman did not have feeds and minerals made specifically for horses. He said that horses were not a big enough market for them but that everything the applied to the feedlots, breeding cattle and dairy cattle would apply to horses. He named off several big horse operations that used Moorman minerals with added Vitamins.
The minerals I feed today are just copies of the old Moorman minerals. All the other mineral Companies like Stockade and Ragland just copied Moorman's formulas. He told me what mineral mix I needed and I just picked it up at the dairy in 50# bags until I moved to Oklahoma in December, 1979.
That mineral had 250,000 units of Vitamin A per pound of mineral mix. It had a Ca:P ratio of 4:1. It had 2% Magnesium and had Zinc in it. The mineral I have now, has only 150,000 units of A so I still give oral liquid A in the winter to some of my horses.
So there, If you suffered through all of this, you have the whole life history of Moorman minerals and how I got to using them.