And for the record I don't think anyone is more interested or involved in nutrition than I am.
That's a pretty bold statement! :]
I just sold a horse I had for a few years who was grain intolerant/borderline IR - I have done tons of research into the best feed options.
As such, I choose not to feed any grains. It's not that grains are 'bad' persay - Just that they can cause issues with some horses. You won't know which horses until it happens, so I choose not to take the risk and feed other products without those risks.
Every horse is an individual also - I feed all my horses the same in varying amounts, but that is because they are all very similar in size, activity level and metabolism. If I had different types of horses, I would most likely have a huge variety in my feed shed :] No one feeding system is going to work for every horse.