Only reason I find to use the tent (BA FlyCreek UL1) w/o the fly is for bugs. On JMT in August, whenever it was warm/calm/dry enough not to need the fly, I went Colin Fletcher. Including in the middle of the beach-formerly-known-as-Lake-Edison. Those were few, but the best nights of the trip. Found I got more condensation on the bag w/o the tent than with it a couple of times: ice at Sunrise/Long Meadow, e.g. Operative word here is "meadow", of course, even well off the vegetation.
Otherwise, with any wind, or highest elevations (lowest temps) generally, it was tent w/fly. Don't think the fly/ground sheet combo would have worked in many spots, with all the intermittent flash mini-flooding going on, even apparently high spots were collecting water. Glad the BA FC floor was so tight several times.
