To Stall or Not To Stall?
Horses are highly social animals who evolved on grasslands. They are built to graze all day, eating small amounts continuously while moving constantly. We've taken them out of this natural environment, thus rendering them completely dependent upon us for all of their needs. In exchange, we need to let horses be horses in every way possible.
Horses turned out with other horses 24/7 find comfort, security, and social stimulation from their membership in a herd (even if only a herd of two), and therefore they tend to be psychologically healthier than horses kept in stalls most of the time. Typically, free horses in a herd develop fewer destructive behaviors - cribbing, for instance, is much more common in stalled horses, who become bored and frustrated and anxious. Being able to graze as a herd on pasture is the ideal solution, but if you don't have pasture available, simply allowing them the freedom of a large paddock in the company of other horses is far better than holding them alone in stalls.
Bear in mind, though, that horses in a pasture or paddock require a sheltered area to escape from the elements. In winter they must be able to get out of the wind, rain, and snow, and in summer they need a shady place to occasionally get out of the sun and escape from the worst of the flies. A run-in shed (that is, a shed missing one of its side walls), with its back wall to the prevailing winds, is sufficient shelter for all but the worst of conditions. Be sure to provide a large enough space so that all of your horses can find shelter at the same time. The boss horse will often hog a small shed for herself, leaving all but her favorites out in the cold or heat. Dividing a large run-in shed into a number of open stalls can help discourage this, since the boss-horse can be in only one of the stalls at a time. Some horses, given the choice, will just prefer to stand out in the weather no matter how nice a shelter you provide, but you must give them the choice anyway.
Here at New Avalon, my personal horses are together and unstalled 24/7, except in unusual situations. During the daytime from late spring through much of the fall they're out on the pasture most of the day, and at night or in the winter they have the run of the barn plus a paddock comprising a good third of an acre just outside the barn's back door An automatic waterer is installed in the barn's aisle in the shade, right by the paddock door. To turn the horses out onto the pasture in the morning I just open a side gate in the paddock, giving them access to a hot wire-fenced alley leading to the pasture. In the evening I call them in and close the gate to hold them in the paddock for the night. I can't let them stay on the grass more than 12 hours a day; they would get too fat, and our limited pasture would get too thin.
My horses are hardly ever held in their stalls, except at feeding time or while waiting for the vet or the farrier. Occasionally I'll close them in for a few hours just to remind them how to stand calmly and patiently if I really do need to stall them. Even back in the hard winters of Pennsylvania I only occasionally needed to stall my guys for the night, if below-zero temperatures or blowing snow necessitated closing up the barn. Horses are a lot hardier than most people give them credit for.
Along with its psychological benefits, keeping your horses unstalled helps your pocketbook and your back, too: no stalling means much lower bedding costs, less dirty bedding to dispose of, and much less time and effort spent mucking out and hosing down stalls (given the choice, most horses will relieve themselves out in the paddock rather than in the barn's aisle).