Finally found that the Stables of Vigor adds 'skills' to any mounts built at this city. They get tagged with Vigorous Mounts and the skills are Free Movement (no terrain penalties), Fast Healing (+6/turn) and High Morale (+200). So don't waste time building until you are ready for those units. It might be worth it to create an Army of these for fast response unit but otherwise, while in a normal mixed army, only the fast healing is a plus for me.