I carry a heatsheet too, but keep the bivy in my jeep. I considered carrying the bivy, but figured that, if needed, I could rig a shelter, using the heatsheet itself as a liner, and, with a fire going, likely stay warmer with the reflected heat, than with it wrapped around me.
Now, that being said, I am obviously planning on building a fire; should that not happen, I still have my military poncho for shelter, and can use the heatsheet for warmth. I think though that the bigger issue would be dressing correctly for the weather. Although I enjoy wool, I almost always hike in synthetics. Simply because they dry quicker. I keep my natural materials for my campground activities.
Again, most, if not all of us here, have spent more than a few nights, or weeks, out in the boonies, testing & trying new gear. with the wealth of knowledge all of us bring, it really comes down to what you're comfy carrying. Some prefer to do more with less, some prefer ease of use. These dont always mutually exclude each other; but, with a wealth of knowhow, I am sure some of you could disappear for a week with nothing more than a wool blanket, good knife, and a sharp axe. Me, I like to make some things easy; firestarting, for example. I CAN do it with a bowdrill; havent in probably 15 years, but know I have. Hence, I have firemaking material everywhere in my kits & on my body.
Shelter, for me, is the same; I can build something, or find something, but the convenience of my poncho works well in most situations. Besides, I have spent many, many nights under a poncho; it almost feels nostalgic Of course, that feeling would quickly fade on about day 3 of a real survival situation for me...but, I havent been there yet, and sorta hope not to go there!