Thanks guys, you are articulating very well the exact debate I've been having in my own head. Michael, funny because while I didn't say so in my first post here I was considering the very idea you propose, a shopping-bag-thin poncho carried in a zippie to layer over the windshirt if need be, there's one in my storage bin somewhere already. And Dale, I agree I wouldn't expect it to hold up to a true downpour nor would I enjoy being caught out like that.
To clarify my question, I'm not talking about an everyday, all-circumstances solution that would work in the Cascades, or here in Georgia for that matter where we can get summer "gully warshers" that dump a couple inches of rain in 20 minutes. But at first glance it seems possible for the usually-dry High Sierras in the summer. I was in Yosemite this past year also for 9 days, same time of year, and got two bouts of light precipitation, neither of which should have been a challenge for the Houdini. Not that there's any guarantee that this year's weather will be the same, it's just my point of reference that started my wheels spinning.
If I force myself to choose between rain jacket or windshirt, only one or the other, I may be better to choose the rain jacket (DriDucks). Last year that is what I pulled on when it rained and I never even unzipped the Houdini from its pouch pocket the whole trip. And my Houdini is not breathable -- maybe technically, but if my face were wrapped in a single layer of what my Houdini is made from I would be dead from suffocation pretty quickly based on my home-brew breath test.
From a weight standpoint the Houdini+uber-cheap poncho is within a few tenths of an ounce of the DriDucks jacket, not an issue if choosing only one solution.