As long as we do it humanely I don't see an issue. We see a ton of piggybacking throughout the animal kingdom from parasites to more complex symbiotic relationships. And then from another perspective, the animal world is insanely cruel to each other. It's just the circle of life. Domesticated horses can/do live happy, full lives. I'm completely against any inhumane treatment of animals we use for utility or otherwise, but utilizing the utility of horses, mules, camels, ect doesn't have to be inherently cruel, especially in comparison to how the animal kingdom interacts with itself.