IMO, the USGI poncho is too heavy and bulky for what it brings to the table, especially if the user is a dayhiker and not a soldier. In the environment you describe, you should have some type of waterproof/breathable shell (even if it is simply the venerable DriDucks jacket, which is pretty darn good for the money). So "poncho = raingear" shouldn't be part of the thinking. Ponchos suck in general as raingar, but they REALLY suck in windy mountainous areas where you need to see your feet as you move through rough terrain.

A dayhiker's emergency gear has to be uniquely light and compact, compared to an intentional overnight backpacker who expects to bivy. So I would say your emergency shelter system should have 3 components:

1) an emergency bivy bag of some sort, like the AMK models or just a big clear bag to crawl into. Weight: around 5 ounces or less.

2) an improvised tarp of some sort, like the 2-person AMK heat sheet (which is 5'x8') tied off at the corners with a sheet bend. If you have the money consider a 1-man silnylon tarp or go whole hog with a small cuben fiber tarp. Add 50 feet of Kelty Triptease cord for 1 ounce. Weight: 4 to 8 ounces (depending on fabric and size).

3) simple, light, immediate insulation from the ground, like the Gossamer Gear 3/16" evazote foam pad. Evazote is fantastic stuff. This pad gives you an immediately clean, dry, warm(er) place to sit out the nasty weather. Supplement it with natural materials, of course, if they're available. The pad is great material for an improvised splint, and it is useful on every hike during rest stops. Weight: 4.5 ounces.

So for around 1 pound or less, you can have an effective, versatile, and compact dayhiker shelter system that weighs less than a USGI poncho alone. Like hikermor wisely pointed out, practice various setups ahead of time and learn how to best deploy this modular set of gear.

This is just a first-pass idea, there are lots of options out there. In particular, explore the cottage gear makers who serve the light/ultralight markets.