Dave,
Most of our trails are multi-use... although we have set aside areas trails that are non-motorized and some that are single track only.
Horse traffic is not real common on our system and we are forunate there as we have clay for soils and a lot of horse traffic would really groove them + we have no $'s for trail maintenance.
I have always said... Horses on our trails.. are nothing more than a mouth, a stomach, a fertilizing unit, and four shovels - digging there way down.... the heavier the stomach area the more efficient the shovels are!