All 5 of my horses are outside. 4 are on pasture. My 30 yr old blind appy, Cheno, was with the herd until this winter. He is currently in a stall/paddock setting due to needing more feed and total access to it without being pushed around at the hay feeder. The only time I shut anyone in a stall is if the weather is horrid, I will shut Cheno's paddock/stall door with him inside. ;) Sometimes I do put the pony in the stall next to him for company. My (2)stalls are 12x16 so he has lots of room. The others have access to the 24x30' open area inside the barn where I've not added other stalls. They only hang around in there if it's really nasty and I hang haybags there ;) Oh sometimes in summer they also hang around inside to hide from the flies and heat of midday.
My horses are out in a paddock 24/7. They have a dam in their paddock to swim/play in, trees for shelter. Honestly, they love it, and I think they would be miserable in a stall. Theres nothing better than watching your horse roll in the dirt after a long ride and seeing the satisfaction they get out of it.
I think I would only stable my horse if I was showing at a national level... I do small shows but rugging them a few days before the show in summer and keeping them rugged in winter is enough to keep them looking schmick. We have a sheep yard that we use if any of them are sick, it backs on to the paddock sot he others can talk to them still, they don't get lonely.