Originally Posted by blackbishop351
Without a zipper, you'd be using a drawstring on the end for entry/exit. That means that one entire half of the bivy would have to be the same girth as the largest dimension of the hammock/user, which requires more material on that end. If you use a zipper, both ends can be form-fitted to the system, thereby reducing the necessary amount of material by quite a bit.
I understand what you are saying now. Great description by the way. Thanks.