According to Dr. Beth's book:
7. Controls or prevents EPSM. Because of the high incidence of this metabolic problem in draft horses, feeding fat instead of soluble carbohydrates could be life saving. EPSM can result in decreased performance, a reduced life span, or catastrophic events such as sever tying up going dow or sudden death.
Later in the book she mentions, "A diet change prior to the onset of any serious problem is much preferred and is part of good draft horse management."
When I contacted Dr. Beth about the diet she it most certainly could help prevent EPSM and there was no sense in not doing it (the diet). Prevention is better than trying to cure. So I do disagree with not feeding the high fat diet unless the horse is showing symptoms.
A horse can have it for a long time before the symptoms begin to show.
Unless it weighs a ton... it's just a horse. Draft horse motto.