My high school's motto was mens sana in corpore sano: (strive for) a healthy mind in a healthy body. That's exactly the goal that horse owners should have for their charges. I'm an advocate of pasture board for most horses because it's the closest thing we can offer to their natural environment. Pasturemates offer social interaction, while constant walking and grazing keep them healthy. As one of my vets likes to put it, "Let your horse be a horse."
A recent article snippet at TheHorse.com offers a few details about a study of stalling horses versus keeping them at pasture. Just another way to improve your broodmare's life!