you need a fly that goes all the way to the ground.I would disagree with that one at least for most of the times and places I have toured. I found that for us on the Trans America and other shorter trips the bigger problem we had was lack of sufficient ventilation, not lack of coverage. Even with a tent with fairly minimal overhangs on the ends and sides that were above the ground by design we needed to use sticks to prop the sides of the fly up to allow more air to circulate under. That was a record hot summer for much of our route that year though, so in cooler wetter weather maybe I would have felt differently, but... While we did tough out a few storms, we spend MANY hot dry nights wishing for more movement of air.
On the TA we tended to spent a lot of nights under town park pavilion roofs so we could leave the fly off. I miss that now that I have a single wall tent.