This is a great thread!
My opinion is that every horse, every situation, and every feeding program is different, and that's okay. Personally, I try to stay away from much grain and sweet feed because I feel there are better feed choices available for my piggies.
The feeds I use and like are rice bran meal, ground flaxseed, alfalfa pellets, and soaked beet pulp. I supplement my two hard keepers with a little bit of Omolene 200 but it's a very small amount just to give them a few extra calories. The bulk of their calories comes from other sources.
I'm a trimmer and the healthiest feet I see are attached to horses that don't eat a lot of grain, and they get a good deal of turnout on varied terrain. Hooves are very important to me, and I think that horses eating less grain "generally" have healthier hooves.
Grains like oats and corn and sweet feeds which are enriched with grain products and molasses contain a large amount of sugar. When the sugar is injested, the body has to release a proportionate amount of insulin, and insulin is somewhat inflammatory in nature. The less sugar floating through the horse's system, the better.
Conversely though, I trim a handfull of horses who eat a fairly large amount of grain daily to keep weight on and they're healthy and fine. Their feet are 'ok' but could probably be better. Some of them are eating around 5 lbs. a day of sweet feed and have for 2 decades and have never had a problem. Again, every situation is different.
In general, I think if you can reduce or eliminate grains and sweet feed from the equine diet, you should. Equines by nature are foragers - grazers. However, it's important to remember that many hard working horses do need extra fat and calories than forage alone can provide. Not every farm is blessed with nutrient rich grass for horses to graze on. Where pastures are sparse and thin, it may be necessary to supplement more. In that situation though, I would reach for the beet pulp, flax seed, or even corn oil before going to a sweet feed.
Great posts everyone, I enjoyed reading them!