I always get into this debate with other horse people. My current barn has the horses outside 24/7 with free access to hay and water and they are put on grassy fields in intervals. Growing up, I always went to barns that had the horses stalled at least at night if not during the day as well.
After experiencing my current barn, I don't think I'll ever stall my horse. It's not their natural habitat. It's isolation and lack of movement, neither of which is healthy for horses.
What are your thoughts?