Personally, the reason I don't like carrying a big kit is because the bigger the kit, the more likely it is going to be left behind when you really need it. A lot of times people get into trouble when they least expect it, like a short dayhike, wandering away from camp to use the restroom, exploring a side trail, etc. Those times are when they will most likely leave things behind "just for a minute", especially if it's heavy or bulky.

I do carry a heat sheet bivy bag on every hike, regardless of duration. If there is a very remote chance that I might be spending the night, I'll pack some extra clothes and a poncho/tarp. If there is a decent chance that I might be out overnight, I might carry a regular bivy. For anything more, I'm probably camping anyway and have a full shelter and bag. But that's inside my pack, which might be put down to take a break or whatnot. I'll still carry the really essential equipment on me.