these single wall tents popular with brits, must be because they dont get the hot muggy summers we get here. Ive always used tent+fly (top part, completely waterproof--main body has varying degrees of mesh)
A non-meshy main body would be horrible in canadian summers, with heat and humidity. Various tents have varying amounts of mesh, so as most of us campwhen it is summer time, more mesh is usually appreciated, and or you can put the main body up and no fly, for max air flow to see the stars.
Im just used to two piece tents, and for my uses wouldnt want a uni body one.
Camping in cold would be cooler or cold in all mesh tent.