I love that idea - I also used the chair kit (Thermarest Trekker?) without a groundsheet for a while. The only problem I had was that I would occasionally end up with some dust or dried mud on the bottom of the sleeping bag, where it hung over the pad. I solved that by cutting a piece of Visqueen (ultralight, back then) slightly larger than the sleeping bag and using that under the chair kit.

Nothing better than sleeping under the stars, then clipping the chair straps together for breakfast in bed (stove by your side, right?)

At the time I was using a tarp, it was the lightest solution for shelter; the groundsheet, tarp, and chair kit together was still 3 pounds lighter than the two person tent I had been carrying. I switched back to a tent when the MSR Hubba solo tent came out. It weighed the same as the tarp setup, offered the protection of bathtub floor and rainfly, and was all mesh except for the bottom two inches which were waterproof. It was as close as I could come to sleeping under the stars, with the bonus of bug protection (critical around my part of the country.) I still kept the chair kit, though.