Where have you been Palomine?? - not reading many threads here, by the sound of it?!
Yeah, I don't believe grain is *necessarily* a bad thing, but generally horses don't need it & if they do need extra weight, high octane calories, there are better, healthier options, such as alfalfa/lucerne, rice bran, etc.
Horses don't do rich, starchy diets very well. They also have a small stomach that empties quickly(approx 15 minutes or less). Cereal grain is generally not - or little - digested in this time in the stomach, so hits the 'hind gut' undigested where it can cause 'acidosis' & other issues. Oats are a bit of an exception, being more easily digestible & lower in starch than other grain. Corn/maize is about the worst, in both those regards. If horses are fed infrequent &/or large meals, such as the norm of hard feeds only a couple of times daily, the problems associated with grain & starch overload are compounded. So I'd also only opt for grain(assuming no better option) if I could feed it little & often, and would ensure it was cooked/processed before feeding, to make it more digestible.