I personally would not treat a horse for a condition the horse may or may not have. Not to mention medications for ulcers are not cheap.
But there are cheap ulcer meds (Corta-Flx U-Gard - Dover Saddlery.). And with scopes running around $300 in this area, it's a lot more economical to put them on a supplement and watch for a change in behavior sometimes. I'd much rather spend $25 on a supplement that does nothing then spend $300 on a scope that shows the horse is clear.
If the supp. Helps then you can scope to check the severity.