Helium II-must have
This layer I find extremely useful rock climbing in the desert or alpine, ski touring, or trekking; this layer I always find in my kit. At 6.4 oz It's too light and compact to not at least have with you. It is the go to layer when I'm not sure if I want to bring a waterproof layer (but can't afford not to), don't have the room for a standard GORE storm shell, or don't want the weight.
This Pertex light weight shell is a perfect solution for keeping it light when you want to stay dry.