Wow, that is a great question! I would say it depends on the conditions and the need. My horses usually only get real baths (soap) before showing. But then again, I have 12 horses and bathing isn't top priority with that many horses! But they do get a lot of currying and grooming.
Some things to consider when looking at bathing a horse:
Bathing with soap will strip the natural oils from the coat temporarily. You can counter act this by using soaps formulated to replace the oils, or for those on a budget, rinse the horse in a bucket of warm water that is mixed with a few tablespoons of baby oil.
Temperature is also a factor, don't chill your horse. If you absolutely must bathe a horse in cold weather, dry them quickly and keep them as warm as possible. Some folks will put a blanket on them until that blanket is soaked, replace the blanket, etc.
Some horses do need more baths. Horses that are prone to things like rain rot will often need betadine baths regularily to keep that under control. Such horses also benefit greatly from regular currying.
Also, especially in the heat of summer, sweat will leave salts in the coat. This can make the hairs brittle and faded. So it is a good idea to at least rinse the sweat off after working them when possible. That's why I do with mine, they get rinsed after sweaty work outs.
Hope that helps some!
