For all their convenience, zippers are heavy and somewhat prone to failure. In full sleeping bag lengths they can often add a full ounce to your sleep system. They can snag and pull on the ultralight fabrics making it necessary to reinforce the area around it, further adding to the extra weight. Lastly, they require draft tubes or some sort of system to cover over the uninsulated area where the zipper is....and of course, it isn't all that uncommon for them to fail. On a camping quilt, we just get rid of it, along with the whole bottom of the bag, which is redundant weight. However, my false bottom bag design provides an opportunity to approach venting a sleep system differently. First, the top drawstring can be used for primary upper body venting by opening it and sticking an arm out or pulling it fully down to the waist. Next, the false bottom of these bags can be spun around to the side or top, allowing heat to escape through the single layer fabric without exposing the body to the outside. It's kinda like sleeping under a sheet on a hot summer night. Beyond this a sleeper can get out of the bag entirely and drape it over themselves as needed on hot nights. Additionally, a drawstring footbox can be added which allows the sleeper to open the bottom to vent and even pull the footbox up, sticking feet and legs out. A side benefit of the drawstring footbox, not related to venting, is the ability to get up and walk around in your sleep system. These tend to add around .5 to 1 ounce to the finish weight. With all of these venting options sometimes it just isn't necessary to use a zipper. It's a particularly attractive route for those going ultralight and/or looking for bombproof gear. However, I do use them on the Vireo which is my full bag and the Tanager which is the false bottom bag with a full zip.