I've thought about this too, in the context of summer hiking in the Southern Appalachians. A rain shell keeps me warm, but I get soaked inside from sweat. Bring the shell, or just rely on my wind shirt for protection in the rain? It's been raining heavily here all day, with steady temps at 60F. So I did my daily walk wearing my UL windshirt over a short sleeve wool base layer, nylon shorts, hemp Tilley hat, mesh trail runners, and Darn Tough light crew wool socks.
My head was dry and happy. My feet were wet but happy. My shorts did a reasonable job of not wetting through, but I can see that after a couple hours of rain they would be soaked - so I will continue carrying my rain skirt.
The wind shirt failed. It wetted through within a few minutes, and after that my arms, hands, shoulders and (soon) my torso were wet and very cold. Now, it's an old single layer LL Bean wind shirt, and has no DWR left at all, so it's no real surprise. However, any DWR will eventually wet through, and I'd be just as miserable even though it might take a little longer.
So the rain shell will continue to make it inside my pack, along with the homemade rain skirt. The Tilley Hat makes a nice addition, since I can see and hear much better with the hat than a hood. Also, too, I will get good sun protection -- which is useful even in the summer, as I'll be in the open enough to warrant some protection. The wind shirt will make the cut based on the expected temps and weather conditions -- in cold weather it's a no-brainer, but maybe not in the summer.