Sorry, but that argument doesn't stack up. The world has massively changed in the last 100 years, and cars now massively outweigh horses in terms of what the roads are designed for. The transition period has long gone, and horses simply no longer belong on the vast majority of roads. Of course they still have the legal right to do so, and until that changes I'm firmly in the camp of giving them as much respect as possible as I do with all road users, but that doesn't mean that I think they belong there.
Horse sh*t may be good for plants, but that doesn't mean it belongs on the road. If a horse rider has to temporarily use a section of road to cross fields, then that's fair enough (much like the rights farmers in tractors enjoy with regards to licensing issues), but otherwise they simply shouldn't be there. A grass track is for a horse, an asphalt road is for a car.
For absolute clarity, anyone who uses the roads without respect for others deserves shooting. Be they horse rider, cyclist, or driver, if you can't respect others and use some common sense when around them then you shouldn't be allowed to use the road.