My windshirt is just that - a SHIRT. It's my spare and easy way to keep me a little warmer. No, it's not an insulation piece, but it will keep me warm enough while moving and might even keep me from throwing on a proper insulating layer at short stops.
My rain jacket is a true shell. If my windshirt won't keep me warm, this comes on. Its eVent fabric is breathable enough to be comfortable to hike in, but only if I'd be cold in my windshell. So they don't overlap, and at less than 9oz I have full weather protection.
In desert climates I've left my rain shell and gone only with my windshirt. Most of the time though, I bring both. When I'm expecting near-constant rain, I'll leave the windshirt at home, instead bringing a proper second base layer to enjoy something dry at the end of the day.
Personally, I don't typically wear/bring fleece in 3-season conditions anymore. A windshirt has proved much better, and with an UL down top, I have more versatility, more warmth, and smaller pack space all at the same weight.