You know, I just spoke with my new farrier about this last night. He said supplements can be good for horses who have a reason to use them - cracked hooves, weak walls, poor hoof quality due to genetics, etc. He told me that horses with strong, healthy hooves don't really need them, though they wouldn't cause harm. When I had asked him about giving my horse supplements because his front left hoof kept chipping this past summer, he said that was due to the form of the foot and how it was being trimmed. Improper trimming, not lack of nutrients, caused the excessive chipping I saw. He said that, for me, since my horse has strong, healthy feet with a healthy frog that giving a specific hoof supplement would be a waste of money. If I wanted to give him a general multi-vitamin type of supplement that includes biotin, I can just for overall health of the horse and fill in the holes from his diet.
Just my 2 cents.