the benefit of having a wee shelter is that u can catch them easier, now that is a v handy thing

. but catching them mite send them thur the poly walls if they r lively, some sheep can motorbike the walls. how about a shed instead, or a field shelter like ones for ponies, it mite not be much more expensive than a polytunnel.
i think pnuemonia etc is common wen stock is kept inside in warm conditions, as opposed to coming in for a feed, but ud have to turf them out if they start hogging the shelter (excuse the pun)
we have alot of trees and all animals make use of them, pigs, cattle, sheep, ponies but it then makes checking the livestock more time consuming as u cant see them at a glance.