Ponchos are great as long as you are walking in a straightforward manner on a trail. When you have to scramble, etc., a poncho can get to be a tad cumbersome...
Also note that ponchos don't work very well in windy conditions. Go for a hike in sideways rain with your poncho....after that you will probably decide to get rid of your poncho and buy a good jacket and rain pants combo.
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz