It's good thinking.
I think that some/many of us use our quilts or sleeping bags for a wrap/cape around the upper torso when it's cold and we are at camp. At least, that's what I do.
That's why I don't carry a down jacket with me. I use the sleeping bag for that use at camp.
But, this blanket with snaps brings some additional versatility, with the down sleeves and leg warmers.
However, this versatility introduces some other limitations, such as it then becomes too small to actually be the sleeping bag or quilt, and so you must then carry this AND the sleeping bag.
So, unless you could reduce the weight of your bag or quilt by the amount of weight in this blanket, you may not be really getting much benefit in terms of weight.
I like the idea. I think it could be further developed to overcome most/all the limitations.