Interesting to read the various experiences; definitely looks like a HYOH thing. I suppose it's helpful that, living where Doug Johnson and I live (Northwest), if it's raining it's usually cold and raining, and when it's sunny it's dry and sunny. This probably makes it more doable. I also do tend to slow down when it's raining, but that's usually because around here it gets quite slick and muddy!
I'll admit though that the one time I really felt stuffy and uncomfortable in my eVent jacket was an early June hike in the Columbia River Gorge. I did a 3000' vertical ascent in 65 degree humid weather that turned rainy (and warm). That was pretty miserable. Still, I doubt my windshirt would have felt much better. I guess I could have just gone shirtless by that point.
There is one other benefit to hiking in cool, humid conditions with the eVent jacket on. I've done a few hikes in 35-45 degree, foggy, calm conditions, and with just my baselayer I felt quite clammy. However, with my eVent jacket on, I felt noticeably drier. This was odd to me at first, but I think it has to do with stopping the constant barrage of water moisture from the air soaking me, and allowing the breathability of the jacket to expel the interior moisture. In either case, that's when I started seriously thinking about ditching the windshirt altogether. Ultimately wind shirts are so light and compact that it shouldn't 'really' matter if I bring both. But on paper, it sure looks more appealing to mentally subtract the 3oz from my rain jacket.
I am coming around to the merits of hiking in the rain with shorts. It really helps to keep the stuffy feeling that can accompany rain jacket/pants combo. Problem is there are no long hemmed eVent jackets on the market, so I have to bring separate 'rain shorts' (aka a swimsuit) to keep my regular shorts dry, adding a hassle factor of changing clothes. Perhaps what we need on the market is convertible rain pants; you can wear them in shorts mode when it's warmer, and then zip on the legs when you stop or when it cools down.
Last comment: I've also considered hiking with a Gatewood Cape and umbrella combo. The weight of the Umbrella is offset by the shelter weight savings and lack of rain jacket. In warm, calm rain I can use the umbrella to allow me to remove the hood and zip open the front a bit. I figure if it becomes too windy to use an umbrella, then I shouldn't have to worry about overheating with the hood up and waistbelt attached--the wind will take care of that. I definitely trust the Gatewood as a poncho in cold, windy conditions; it offers very good protection when using a waist belt. I guess I'm still building up my confidence in the Gatewood's high-wind performance as a shelter above treeline--where I like to camp. I know it's been done successfully, and it's wind protection is one of the features of the cape. But still...I just don't know. I just have have more confidence in my eVent jacket combined with a Golite Hex 3 (soon to be replaced by a Shangri-La 1) above treeline.