I actually got heavier going to a quilt by a few ounces, I already used sleep bags unzipped as the restriction of being zipped in makes it hard for me to sleep. A quilt solved a lot of problems for me and I sleep better, which is why I will keep using it.
For people who are happy with their bags and only want to save weight, I think getting a second bag for warmer weather is a better us of money. Good or bad sleep carries forward for the entire day and the effect of poor sleep is cumulative.
I find my floorless shelters to be simpler than most free standing tents I have used, I have a spreadsheet though, so might just be broken upstairs.