As with humans, I don't think you can make huge, sweeping disclaimers about one feed program or another without evaluating the individual horse.
While I agree that a non-working "pet" horse of most breeds can get by without grain (concentrates) and subsist on hay only... That might not be the route to go for a Grand Prix dressage Hanoverian in intensive training.
Nutrition CAN be a science. You can break feed down in to calories, fats, fibers, vitamins and everything in between. It very well may be possible that the available hay in your area might lack in protein or something.
When we make blanket statements like "I'm feeding just plain oats and my horses are fine..." Does this mean you're feeding a negligible amount per day such as a cup or are you feeding a substantial caloric amount such as four pounds?
Personally I don't mind "feed from a bag" because it is formulated for my horses that get zero pasture and do moderate work to make sure they are converting calories into muscle... You can contact the companies' nutritionists and they can take your hay analysis and their feed analysis and help you maximize your horses' performance.
But sure, if you've got pets standing around they will look relatively healthy if most of their basic needs are met. Most Americans can get by eating fast food and can live normal, healthy lives. But I'm guessing people who are heavy into athletics or need to watch their appearances like actors can't get by with the same food choices. Many of them will choose supplements and such for the convenience of making sure their dietary needs are met after going to the gym regularly.
By the way, my horse gets far, far more regular exercise and attention to diet than myself... Isn't that the case?