My insulation layer is different for summer backpacking and summer day hiking. For day hiking, I have a system that works well for me: a base layer synthetic T-Shirt, nylon shorts, a fleece sweater (old model Patagonia mid-weight) and a wind shirt (or rain jacket). I need the fleece for mountain tops (it could be 85 and nice in Seattle, but rather chilly on top of a windy peak next to a snow field).
For backpacking, I replace the fleece sweater with a Cocoon pullover. I really don't need the extra warmth during the day (or even during breakfast), but it is warmer for the weight. The extra warmth allows me to carry a lighter sleeping bag. In other words, the pullover shaves a few ounces off of my system. I often hesitate with my Cocoon pants, since I'm usually just fine with my rain pants for morning warmth.