You make well reasoned points, ones that Ray Jardine also promotes. In normal conditions I think either of those alternatives make sense. After all, 6oz of down is a lot of added warmth!
For me, the problem has more to do with wind blocking than insulation. I've been colder in high wind wearing thick fleece pants, and warmer when I switched to thin summer pants plus rain pants. And even though sleeping bags often use wind-resistant fabrics, there is something about cold, blowing air directly circulating around a thicker sleeping bag that is colder for me than having that extra layer of fabric (the bivy) on a less thick sleeping bag. As a quilt user, a bivy really helps as well.
I use a bivy for other reasons too, though. It's an added barrier to bowing rain and dripping condensation, and it lets me sleep under the stars when conditions permit while knowing I have a shelter to crawl into if I needed one.
I live in pretty rainy conditions, so I use a 2-layer wp/b bivy which gives me the peace of mind that, if stuck in a storm, I've got an emergency bivy. But I tend to hike in conditions where a double-wall tent may make more sense, so a bivy in a tarptent is a lightweight way for me to achieve similar results, and benefit from a wider range of comfortable conditions.
As always, I think one has to evaluate one's expected hiking conditions and how many safety nets one wants to put between ones self and hypothermia.