It is my understanding and it makes sense to me, that loose footed sails maintain a better foil shape at the foot, making the sail more efficient. Also, it seems as though an older sail with a possibly shrinking bolt rope in the foot, may be harder to stretch into better shape with the outhaul, if it's a captured foot, affecting the entire sail shape . Loose footed sails are a little less expensive and quicker to dress or undress and bag.
-Will (Dragonfly)