I think horses should be pastured. I've noticed a huge difference in my own horse's happiness when I moved them to an area with pasture. Even if all they get is a large dirt lot, it is better than a stall.
I work for a barn that keeps all their horses stalled (10 horses). Almost every horse on the property exhibits stereotypical behavior. Some crib, some pace, some stand in the corner and just look depressed, some kick the walls constantly. Most of them have issues with colic. At least 3 of them have swollen legs from stocking up or kicking the walls.
A horse was designed to move- it keeps their feet healthy, their GI tract healthy, and keeps their mind stimulated.
I would rather sell my horses than keep them stalled. If you locked a child in it's room all it's life it would be considered child abuse. I consider stalling abuse unless that horse is out working for 3 hours a day or doing something physically exhausting.
Growing up I didn't know better, as that was how horses were kept. Now I do. Unless there is severe weather or the horse has a bad injury, or you are at a horse show/camping trip, I do not believe in stalling.