This thread is an interesting excercise in how we percieve clothing items. This sort of itemizing has been the real learning curve for me in learning how to really get my pack weight down and stay cozy.
If you think "long sleeve shirt," rather than "jacket," then a windshirt occupies a different space in your clothing palette.
I get into what I call clothing "brackets," where the gear (read "investment") I have is on either side of a particular layering option. I have a Marmot DriClime windshirt that isn't at all ultralight (16.4oz/465g). It can protect like one of the ultra-light windshirts, but also has an insulation layer and some DWR too, so it becomes long sleeve base layer and wind/light rain protection at the same time. Typically, when it gets wet in my climate, it gets cold and windy, so I would want a little more than just the single layer wind shirt anyway. It can be added over a silkweight tee or worn by itself and it vents well enough to become a layer under a raincoat.
I would see the windshirt alternative as using a short sleeve tee, a long sleeve half-zip base layer, and the windshirt to work with-- really the same as the Marmot shirt, but with a "removable" insulation layer.
In my climate (Pacific NW coast), I see a true ultralight windshirt as more of a dry climate option. I could see it being more useful in areas like the Sierra or Rockies, when it might get colder or windier, but not wet. Sun protection is an issue in the drier areas too, where you could be getting a sunburn at the same time the cold wind is sucking the life out of you. Again, im my climate, if it is 45F, it is also raining or just did, or is just about to, and the brush is wet with rain or dew. Cold and humid is the issue here, where hypothermia can come at a higher temperature from being soaked inside and out. My point is that when you would reach for a wind shirt here, you would be reaching for something more waterproof anyway, so the windshirt isn't as useful.
On the bottoms, I layer shorts and raingear (Marmot Precip top and bottom, BTW) for summer, replacing the shorts with zip-off's for spring/fall and adding polyester long johns for winter. I'm working DriClime pants into my winter gear and the plan there is to wear them alone or layer them with long johns in colder weather. I could wear the rain pants over both or with one or the other-- the DriClime pants and/or the poly long johns. The long johns make for good sleeping gear too.
It's quite a chess game-- figuring out effecient layering vs. weight vs. perception.