My boy is white all along his back and in the summer he likes to roll around a lot in the dirt. Then, when I ride, the sweat really brings that dirt up to the surface and gives him a grimy spot at the rear of my seat area, under the saddle pad. I'll often blast/hose this off after riding (sometimes daily). Although, I don't think it would be good to bathe a horse daily with soap because it removes the natural shiny oils from the coat (like Blu said). Could dry out his skin.
No bathing everyday is ridiculous!! It will completely strip the coat of the natural oils that the horse needs! I only bath if I take my horse to a show, or the occasional one in the summer if it is really warm :)
I agree with everyone - that can't be healthy. We hose down every day in summer because they get so sweaty, but never use soap more than about once a fortnight. If you were showing ALL THE TIME, maybe more, but I'd still limit it to once a week max.