It's not so important for the Arabians because they're air ferns, but my TB came home extremely skinny and I've found it to be as close to a magic formula as any of the high priced 'complete' feeds.
That's why if I were to get my feed custom blended, the first and largest ingredient would be beet pulp, second would be rice bran.
It's true the better the feed, the less you have to give them.