Find Backpacking Quilts with the features you're looking for.
How Backpacking Quilts WorkThe idea behind backpacking quilts is that when you're in a sleeping bag, the sleeping bag underneath you is being crushed down by your weight. In a sleeping bad the thing that's keeping you warm is the loft, or thickness of the sleeping bag, meaning that the more loft there is between you and the outside world, the warmer you're going to be. So when the sleeping bag beneath you is being crushed down, its ability to keep you warm is drastically reduced. So why not get rid of it entirely, and save the weight?
The problem with this is that there's the possibility of the sleeping bag coming off to one degree or another during the night, making you cold and probably waking you up. Which isn't any fun. There are a few different ways that backpacking quilt manufacturers address this, but two of the more popular are to have either straps that link the two sides of the quilt together, underneath your sleeping pad, or else have excess fabric on either side of the quilt, which you lay on, holding the sides of the quilt in place.
Why Use Backpacking QuiltsThe main reason you would use a backpacking quilt instead of a normal lightweight sleeping bag is the weight that can be saved. By removing that excess fabric and fill material you're significantly lightening the weight of your sleeping bag.
Another good reason to use backpacking quilts is that you can more easily ventilate your sleeping bag should the bag be too warm for the weather you find yourself in. This gives the sleeping bag greater versatility, and makes for more comfortable nights in warmer than expected weather.
Potential Drawbacks of Backpacking QuiltsThere are several potential drawbacks of using backpacking quilts that you should be aware of. The first is that as mentioned above, there's the chance of something coming loose and your warm cocoon of a sleeping bag springing a leak, letting cold air in and hot air out. But this really depends on what kind of a sleeper you are. If you could be mistaken for being dead while sleeping, then this probably wont be a problem, but if you thrash, it might be worth trying to test a backpacking quilt before you commit to buying one.
The second drawback, though you might not see it that way, is that backpacking quilts don't come with hoods. A toque or some other head warming device works as a replacement, but I personally keep my hair short and have always appreciated the extra warmth that a hood provides for my head. (Check out the Nunatak Catabatic SL and the Therm-A-Rest Haven 20 to see two exceptions to the no hood rule)
Find backpacking quilts and a lot more kind of sleeping bags over at the lightweight sleeping bags list.