I've slept outside with nothing more than a foil blanket on randonnees, but I like to be a bit more sheltered on a tour.
And speaking of rain, in the week or so before the 2007 PBP, and during the PBP, there was a lot of rain in the St. Quentin area, and further afield. The campground where many of the riders were staying turned into a muddy bog. Our tent was pitched on an ever-so-slightly higher bit of ground so it wasn't quite as boggy as some of the surrounding areas, and we discovered that our tent does quite well in the rain.
Very, very glad we had that tent then!
But I seem to attract rain when I travel. It is the rare tour where it hasn't rained at least once. And most places I've camped have had mosquitoes.