+ easier to get into bivy
+ not wasting insulation being compressed on back
+ utilize hood in existing insulation to serve as sleeping hood
+ allows use of existing insulation thus allowing you to get a lighter bag and pairing it with existing insulation
+ can be used with sleeping bag to increase temp rating for colder weather.
+ easier to adjust temperature by allowing in air when warm
+ for those that do not always sleep on their bag, easier to turn with seperate hat + quilt without condensation build up
- no hood and need to wear hat
- potential drafts
+ no drafts
+ full zip bags can be unzipped and used "like" a quilt on hot days
- heavier than quilt+insulation layer quilt
- wasted insulation on back
* half zip bags are not as versatile"
This is a good list, only I would add/modify a couple points:
+can be tightened closer to body to minimize internal air space and thus heat more quickly/efficiently
-no hood and need to wear either a hooded jacket or layered balaclavas or an insulated balaclava, all of which add some weight compared to a mummy bag and a non-hooded jacket
-potential for drafts, which can be minimized by using a bivy bag, which adds weight
+can be used without a bivy unless using an under-sized tarp in rainy weather (example: in the desert, in most seasons you can pretty much count on it not being very rainy, so a bivy would be overkill for rain-splash protection alone and since sleeping bags zip up, you certainly don't need a bivy to stop drafts)
+side sleepers need not worry about cold drafts when they turn over in a bag
-side sleepers do need to re-adjust the mummy bag head hole when they turn over so they're not breathing directly ino the insulation for half the night
-most any bag, especially one with a hood, is overkill in heat of the summer in regions where the lows stay above 60 for most of the night
Overall, this dilemma has frustrated me recently when I decided to make a few MYOG quilts. I'm a side sleeper so I'm not using the argument, "the bottom insulation in a bag gets compressed." That simply doesn't apply to me since I have only successfully slept on my back one or two times in my life, and that was only because I was very ill at the time. With my quilt experience, I found that, *speaking for me personally*, the lack of a zipper on a quilt is downright stupid (BTW- zipers need not be heavy: a 6' length of #3 coil zipper weighs less than an ounce).
In minimizing drafts, I found that straps help--especially if you leave them long enough to go underneath a sleeping pad--but drafts still found their way in, when I was using my quilts, so I found myself making a bivy (5.5 additional ounces to my sleep system in cold weather) to minimize the drafts. What I don't really understand is that when using a synthetic quilt with, say, a momentum90 shell, what's the real need for a second layer of momentum90, the same fabric, to stop rain spash? It seems to me that the bivy in this situation would only be for drafts, but it would actually be needed to minimize drafts, compared to a sleeping bag, which doesn't need a bivy to stop drafts. Doing the math, I found that the extra weight of the bivy negates the weight savings of a quilt! That is, a 15 ounce quilt + a 5 ounce bivy weighs 20 ounces total, which is the weight of the WM Summerlite, but you still need an insulated hood for your quilt system to be comparable with the Summerlite and the hood adds even more weight.
Further, in warmer weather, *I personally* found that if I were using a quilt to stay warm, but still be comfortable, the necessary girth to use it comfortably (read: wrap it loosely around my body without the straps cinched down, and not cause me a headache every time I flip from my right side to my left side by needing to utilize prostrate acrobatics to re-position the quilt) would be 57"+ through the torso, quite a bit larger than the quilts currently being produced by companies like Nunatak, Jacks R Better and MLD, which weigh 11-15 ounces and have <48" widths through the torso. 57" is the girth of some sleeping bags, though! So why not just make the "quilt" wide enough that it can wrap around me totally, and add a very light zipper at the edges? This set up would still require an insulated balaclava or a hooded jacket for head warmth...but like I pointed out in the negative aspects of a mummy bag design: side sleepers need to perform a bit of acrobatics at night in order to re-position the face hole of a mummy bag when they turn from left to right side, so they're not breathing into the insulation. So a hoodless bag/quilt hybrid like I have described, plus an insulated balaclava, one that maintained a good neck seal, would make sense to me.
To take that one step further, the best of both worlds (again, this is *for me personally*) would be a "quilt" that's wide enough for good coverage, with a full zipper, AND some sort of strap system so I could vary the girth to minimize the internal volume of dead air space.
Edit: I just read through the most recent posts (something I should have done before typing a reply to the thread), and it seems I've repeated much of what Inaki Diaz and Allison have said. Oh well, I guess that's just further backing to those ideas.