I agree that you wouldn't want to feed a horse straw in normal circumstances. But, there may be cases when it might work.
Horses are better when they are eating a diet of roughage on an almost continual basis. They are made to be grazers eating for large portions of the day. We (many owners) can do them a disservice by two grain meals a day with a couple flakes of hay each time. While they are OK on that, it's not really their make up.
So consider a horse that is overweight or has ulcers. If you give him enough hay to eat all day long - as he would grazing, you may be overfeeding and possibly creating additional physical problems. For example, I had a horse who had ulcers and really needed to have something in his stomach at all times. However, he also had hock issues and didn't need any extra weight. He might have been a good candidate for adding some straw in for "munching" during the day IN ADDITION to his normal grain/hay diet.
I'd NEVER do it for money reasons, and I'd only do it in specific circumstances. But, it's good info to know IF the right situation ever came up.