It is a daring departure from design norms for MacPac to show off a sleeping bag without a zip. Granted, a few expedition sleeping bags are designed like this, with triple or quad muffs, but obviously, even with good and snazzy looks, any bag sans-zip is going to be a new and possibly irritating experience to wiggle in and out of compared to the ease of just running a zip open and closed! I couldn't imagine myself using one. Never found myself wanting anything other than a full length muffed zip that never tangles or breaks. As for saving weight without a zipper .... drrrrrft.
Adding another 100gm of fill defeats the design purpose. If you need more fill/loft, you move up in the sleeping bag range.
It's always beneficial to actually
visit a Macpac store and try out a sleeping bag rather than speculate online about features (or lack of such). No doubt the
Firefly has a ready, waiting and enthusiastic market, just that it is obviously not going to be suitable for everybody. Visit a store and examine all the sleeping bags for size, fit, features, loft and especially weight.