My 3 season tent is significantly warmer with the fly on. It also makes a easily felt difference if we zip up the fly completely down to the bottom or not. The tent has vents on top we always leave open, but even with that crossflow, it easily is 10 degrees warmer in there.
This August we used the fly every night while on the trail, finding moisture on the inside of the fly just once (Fish Valley, no wind night). We did sleep without tent two nights before and after the hike, and those felt significantly colder, even though we were at much lower elevation during those nights (Portal and Tuolumne Meadows).
vents open, fly unzipped on one side (can be open on both for even better ventilation before you have to pull it off)
