I go barefoot because I believe it's healthier for their feet. I agree that you should do "what's best for the horse", but also believe that a lot of people are uneducated about the benefits of going barefoot and are very set in their ways with shoes. Very few horses actually "need" shoes - those would be the horses that really cannot carry a boot, are covering terrain too rough for a boot, or have hoof issues that require a boot (though my farrier did an incredible job correcting my horse's feet with great barefoot trims when a "traditional" farrier said he would need shoes).
Hoof protection is generally necessary in certain terrain. As many people have pointed out, horses have been shod for centuries in an effort to protect their feet from the terrain that we've asked them to travel on. Between domestication and the tasks we as of our horses, they will need this protection every now and then. Because my horse does not need protection except on endurance rides, when I'm asking him to travel over sometimes rocky terrain and do so quickly, I prefer to keep his feet barefoot and healthy.
Far fewer horses "need" shoes than people believe - and barefoot advocates often find themselves in the minority when it comes to hoof care.
My farrier had excellent training and first began as your typical farrier. However, when she completed her training in barefoot trimming, she more or less became a barefoot-only trimmer because of how much it really does support a strong, healthy hoof. She still has her anvil *just in case*, but hasn't touched it in years. In fact, though rare, not being able to go barefoot would be a deal-breaker for her when buying a horse because that would only be because the horse has major hoof issues.