Something is always better then nothing though.. if space is an issue for you though, and you stil want something to go with you could get some Emergency Survival Bivvys to put in your car, and if you don't think it would be big enough you could add a Two Person Emergency Survival Blanket in there too.. if you want a down bag that can compress to a tiny ball for yourself but don't want to shell out the change, you could make one (I remember seeing a websight that showed you how about a year ago) that is custom fit to you.. and too, there is always shareing a blanket in the back seat, a surpluss wool or wool emergency blanket would work, just lay it flat in your storage area like your trying to protect the car from spills and stuff and it takes up almost no room.
Don't take this the wrong way, I understand what you were saying and I agree with it, I'm just trying to state some options for people that can't fit the kitchen sink in the back of their economy car (like me)