No disrespect intended, but this is 100% false. A good, quality quilt is every bit a swarm as a bag and additionally is alot lighter. It's all in the implimentation. Somene who is not versed in the usage of a quilt should definately stick with a conventional sleeping bag. I have switched over to a quilt, bivy, tarp set up along time ago and would never look back. That being said I own some top of the line sleeping bags and enjoy them, but they fill a niche for only the most extreme of conditions.
One of the main misconceptions out there is that UL backpackers suffer or take risks. This is also 100% false. An ULer looks at their gear as a system in which each item is to be used to compliment the others. Why pack layers of clothes for warmth and then carry a bag rated to 20 degrees and not sleep in the layers that you brought? I see tis time and time again. For the most part in the Sierras, 3 seasons out of 4, a 30 degree quilt is way more than enough when used in conjunction with your layering system. A quilt used with a waterproof bivy and tarp is sufficent tokeep me warm and dry through the most difficult of weather. The whole packages weighs in 40 something ounces.
Nunatek makes awesome, high end, down quilts, Kifaru has a really promising synthetics, high end. quilt. I sometimes use my Snugpak jungle bag which is a reasonably priced synthetic bag that can be unzipped and used as a quilt.
ULing isn't for everyone, but I've been doing variations on it for years and have never once felt unsafe or under prepared.
Nessmuk, Muir were ULer's and they seemed to make it just fine.