Does it make sense to buy a jacket for times when you don't actually want a jacket? In a word, yes. When you're going light, the forecast is decent, and you're tempted to leave your storm shell behind, pack the Isotope. At five ounces, it vanishes until you need it, but in an emergency it will keep you dry. The jacket's gossamer ripstop nylon fabric with microporous coating isn't designed to withstand heavy rain all day, but it had no problem fending off several hours of intermittent yet steady drizzle on a camping trip in Wisconsin. 5 oz (L); sierradesigns.comBonus: Five ounces includes a hood!Bummer: The billowy cut easily glides over bulky midlayers, but the excess fabric flaps in the wind.