Q) Why baselayer + windshirt + down (in that order)?
A) This arrangement is ideal if you're breaking camp in the morning and know the down is only going to be worn for 20 min or so. Once you get warm enough, you can stow it and keep rolling without additional fiddling with layers. Around camp in the evening etc, windshirt on the outside is ideal for max warmth and to protect the down garment.
Q) Why windshirt next to body?
A) This setup would be most suitable for physical bug protection, where you want a physical barrier, but wearing a baselayer as well may be too hot. Some windshirt fabrics are nicer than others for this. A lot of 'trekking shirts' (ie. RailRiders) are similar material as a windshirt. If a windshirt is too 'plasticy' they it won't feel good.
Q) Why a windshirt at all?
A) I like using a windshirt, even for PNW use alongside a traditional WP/B shell. Almost never do I take it instead of a WP/B shell although that may be suitable for some locales (ie. southwest). A windshirt is a lot more breathable than a WP/B jacket, so it's great to wear all day in windy/chilly conditions. While a WP/B shell could be worn, the windshirt is far more breathable so you stay more comfortable and you divert wear/tear from a pricey WP/B shell - especially while bushwacking. It's just much nicer to wear in non-raining conditions so it's worth the 4-6oz considering the amount of use it sees.
I really like this one (MEC RD Windshirt, 5oz, $78). A bit heavier than some, but all the weight is in the material and cut, so it's more durable than most with a nice long torso, great hood and nice long sleeves. I added thumb loops to mine.