Here is a summary of what I consider the pros and cons of quilts vs full bags vs top bags vs half zips etc…
Bags:
A fully zipped bag will be the most versatile. A semi-rectangular bag can be fully unzipped and used flat like a quilt, or just zip up the foot and use it like a boxed-foot quilt (eg Nunatak Arc), or zip it all the way to stop drafts and make use of the hood. These also weigh the most of all the options available. Comfort level is generally high since you can use it just about any way your heart desires.
A mummy bag with full zip is the next most vesatile. It can be unzipped and used like an Arc quilt, or partiall/fully zipped and also has a hood. Very slightly lighter than the semi-rectangular, but generalll a bit warmer for the same amount of down due to the smaller foot box and less space to heat. Either of these types of bags can have attachement straps added if you want that kind of a sleep system option. Comfort is also very high as it can be used as a quilt for resltess or claustrophobic sleepers.
Add a differential cut to you mummy to increase the down efficiency even more.
Plain quilts and closed box foot quilts are next in terms of versatility. If the plain quilt has 'wings' or other attachment systems, or a way to cinch down the foot into a 'box', then warmth can be pretty good. These weigh less than the bag options, but you will need separate insulation for your head. Having the head insulation worn separately is probably more comfortable for a side or stomach sleep (or tosser), but makes no difference to a back sleeper. A bivy bag may also be needed in very windy/cold conditions.
Same goes for the closed box foot bags, but again there is a slight weight saving over a flat quilt. Otherwise the pros and cons are the same.
Top bags are the least flexible sleep system, but also the lightest. If you are a dedicated back sleeper, the fully zippered top bags are an excellent choice, and if you are a hard-core ULer, then a half zip or no zip may appeal to you as you may find the weight savings worth the reduced flexibility. Top bags would be very difficult for a hammocker to use.
What I see it boiling down to is that an increase in flexibility will either weigh more, or be less warm for the same weight as a less flexible system. So our choices will depend on how much flexibility we are willing to sacrifice to keep our pack weight down without freezing.
To me, it's not a big deal. I don't mind carrying an extra ounce or two to have a zipper and hood, but I don't need down underneath me and I don't need room to spare. Then again I don't fidget and I sleep on my back, so a top bag is perfect. A fully semi-rectangular bag is also 'perfect' for sleeping in, but since I don't need that much flexibility, I choose not to carry the extra weight. It wouldn't cripple me if that was all I had available, as the total weight difference is really negligible in the overall scheme of things.
So to answer the original question, I wouldn't say that "Quilts AREN’T as Comfortable as a Sleeping Bag", rather I would say that quilts are just a little less flexible than a fully zippered sleeping bag, but also a little lighter provided you don't need to carry an extra bivy bag or down hood to make it work.
What are others opinions? This is obviously not addressing how much fill/layering etc…as that is really a different issue that depends on the 'style' question.