As a former sheep dude who spent some time thinking and reading and talking about designing barns for health and clean wool production (before I realized that I could go broke almost as fast if I just hung around on FR - and I'd be warm, dry, and clean ...)
Where was I, oh yeah: Some pens can spread disease. Some combinations of bedding and ventilation can compromise the ovine lung (you need air to disperse the inevitable ammonia vapors.) And some penning helps ewes and lambs bond.
Then again, IHS says that He is the door through which the sheepies go in and out and find pasture.
Then yet again, one of the great sights of intensive management sheeping is when you open the lambing pen and a lamb comes out onto the grass for the first time (not counting when it was actually born.)
While it would be messed up for a ewe and her lambs to prefer their pen to the outdoors, still good barning and penning can provide safety, comfort, and health, all their lives.
But to abandon the metaphor, Leaving aside my (so to speak, deductive or a priori) disagreement with the Catholic (as distinct from ROMAN Catholic) Church being just one pen among many, I think that human type personnel need "societies" in which they can't necessarily always have their own way. And so we have the wonderfully self-contradictory phenomenon of "non-denominational" entities which form associations.
But to try to bring at least this minor point back home, you called Donatism a "pen". Since Donatism is kind of an ecclesiological heresy, I want to argue that it's an unhealthy pen, one which encourages the sheep to reject the grain if the guy who feeds it isn't sufficiently holy. A sheep could starve surrounded by good grain in such a pen.
The lord in the metaphor didn't talk about going in and coming out..
He was concerned about getting them out.. but being sheep they could wander back in, I suppose.. The lord Jesus never used the word heresy or even developed the meme of it.. Since theres no ONE pen the inhabitants of one pen would naturally see the inhabitants of another pen as heretics.. Since the walls of the metaphorical pen are dogma, ceremony and/or tradition and possibly intimidation.. Its easy to lose track of what we are talking about here..
Starving in the pen is a good point.. The wisdom would be get out of the pen were there is much food that don't smell of ammonia or shut up and eat the grain you are given.. The lord reacted strongly to the pen of the synagogue..