Thank you all for your input.
An integral component to a layering system, it's primarily for increase warmth over a base layer or base+breathable insulating layer. Today's DWR finishes are generally effective against mist and drizzle but not full-on rain and while in theory a WPB jacket does the same thing in practice I have yet to use one that doesn't become a sweatbox while hiking.
As the name implies, they're especially effective preventing (convective) heatloss from wind. e.g., cycling over a mountain pass you generally arrive hot and sweaty then face miles of downhill at much higher speeds and a much lower work rate, perfect conditions for hypothermia. Donning a shell at the top is SOP.
Hiking doesn't have such dramatic transitions unless you're popping out of treeline into a windy alpine zone. But a simple rest stop can also bring a fast chill and 3 oz in you exterior pack pocket can make a huge difference.
Rick, you mention 3 specific scenarios where a light DWR wind jacket is appropriate.
1. transitioning from uphill to downhill while cycling
2. popping out of a treeline into a windy alpine zone
3. taking a rest stop
In each of these scenarios, wouldn't a true waterproof jacket also perform the same function with minimal risk of getting soaked from sweat? I could see maybe #2 still being problematic, since you're still hiking, but #1 and #3 are much lower output activities.