Your really can't compare the weight of a single-walled tent (Double Rainbow) to a double-walled tent (Spitfire 2).

In the event of a driving rain on a cool windy day, closing the single-walled Double Rainbow would result in massive condensation problems. The double-walled Spitfire 2 with a full-length fly, even with all doors closed, still has substantial ventilation through the bottom of the rain fly and the hooded fly vent at the top.