Part of the reason I don't carry such items is that I'm comfortable in my environment and my abilities. In my daypack, I have an AMK bivy (the Gen 1 version), a 2-person heat sheet, and some plastic trash bags. I figure it's enough for a decently comfortable night, when worn over my clothes. I usually have extra clothes (usually a fleece vest at the minimum), so I have insulation there.

If I'm going for a dedicated camping trip, then yeah, I have a tent and sleeping bag.

My exception to this is my bag I'm setting up for Alaska. It'll have my 1-man tent and my sleeping bag, because I just anticipate harsher conditions up there. While a bivy and trash bags may suffice, I'd rather have the security of a bag and tent.

As for why we keep our kits light and small, it's 2 reasons: easier to bring, and easier to carry. You mentioned you keep your kit under 10lbs. I don't know how you keep it so light - mine runs about 15# all told. But IMO, you seem guilty of the "keep it light and small" mentality too. My question is, why do YOU keep it light and small, and what do you leave out to keep it that light?