The arguments here made sense, but every time I left it home I regretted it. It had something to do with walking in cold moderate to high winds at high altitude in strong summer sunlight. A breathable wind shirt kept the sun at bay and cut down on the chills without putting a fleece top on over a polyester polo. You can argue about the reasons for this, but the outcome was consistently the same.
So I ordered the lightest windshirt I could find at the time, a Montagne pull over. It has a zip chest pocket and two zip vents, one on each side of the tummy, that became a kangaroo pocket with the addition of a small swatch of mesh and a little sewing. I don't care if it weighs a bit more than 2 ounces and lacks 'deluge' DWR. Would not want a 10-20 denier fabric for a cover that takes this much wear and tear, and it makes me much more comfortable in open exposed areas at all altitudes. And when the weather gets nasty, there is the WPB rain jacket that can keep a fleece top dry and me warm.
Also agree that when the DWR and/or calendaring is heavy enough to greatly reduce the breathability of the fabric, it will also greatly reduce the range of situations where the wind shirt is useful. It would be nice to have a gossamer material that breathes like open weave fabric and is functionally waterproof. Our tent, tarps, tops, pants, etc. would all be made of it, but we're far from it.
