Sweet feeds, and to a lesser extent, grains, contain high levels of NSC's - Non-Structural Carbohydrates. The reccomended MAXIMUM for horses is >14%. Research has shown that most commercial sweet feeds and grain-based feeds contain 20%, 30%, even 40% NSC's.
High NSC levels have been linked with metabolic issues ad other issues such as laminitis, founder, tying up, insulin resistance, cushing disease, etc. Basically, it is like feeding a horse junk food. Their digestive systems are not made to digest such high levels of sugar and starches.
Many, many horses spend their whole lives eating massive amounts of sweet feed and seem none the worse for it. However - That is generally because horses don't live long enough to outwardly show any damage that may be caused by an innapropriate diet.
There is NO need to feed sweet feed, or even grains. There are many viable feeding options that are more suited to the horses digestive system and often more cost-effective than commercially produced sweet feeds.
I recently switched all my horses to a no-grain, no-sweet feed diet, as my main horse was tying up as a result of the sweet feed I had been feeding my other horses for years. They have ALL been doing better off the sweet feed.
I use speedi-beet as bulk - To fill the belly, and also supply some energy. For my show horse, who is in hard work for periods of the year, I add copra - A coconut meal high in oils which provide energy, but is still very low in NSC's. I also use white chaff and meadow hay to make up bulk if they are off pasture for a night.
Other replacement feeds are things like flaxseed, micronised lupins, rice meal, oils... So on and so forth. It is easy to get the energy you need without all the junk-food extras in sweet feed.
My horses have an iodised salt lick and a mineral block in the paddock, and don't need feeding at all most of the time, and when I do feed them, it is with the above mentioned grain and sweet feed free diet. They are all barefoot, fat, healthy, shiny, and full of energy.