Actually, the "detached" UQ, hammock and top quilts, not to mention hammock socks, do constitute a modular system. You pick your hammock with or without a bugnet depending on the bite count. Pick your tarp, big one if it's the rainy season, small one if it's not. Depending on the temperature, select from your vast quiver of UQ's and TQ's. And if camping in Minnesota wintertime, the hammock sock.
You go prepared for the conditions at hand without lugging any extra weight that you don't need.
Now I will admit that when I first discovered hammock forums and hammocks that the idea of "half a quilt" under me seemed, well, half cocked! However, I soon learned to trust in the wisdom of these good folks here and went of half cocked myself. To my surprise, I've never had a problem with the partial underquilt concept and love the fact that it saves weight (and money too as I make my own).
Of course, there are many that do prefer a full underquilt. But, I think the majority of backpackers are partial to the partial quilts (I just couldn't resist).
"Life is a Project!"