I think a poncho is invaluable, but not just for personal protection from rain.  I have a coated nylon dark blue poncho ($15) that I keep rolled up in a 2x7" roll in my pack.  It will cover my pack if needed, it will serve as a ground cloth, shelter, shade and of course keeping me fairly dry.  I am not worried about gortex or breath-ability as it is pretty open and breaths on it's own somewhat.
I also keep a few of the $.88 walmart ponchos in my pack as well.  They have served as rain ponchos for my family, as ground cloths under the nylon poncho when used as a rain fly.  They are one time use, but very convenient and take up so little space.
I have not splurged yet on a fancy sil-nylon one, but would love to have one at some point.  The one I have works for what it is needed for and that is emergency shelter, whether that is rain or sun.
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"Do what you can, with what you have, where you are"  Theodore Roosevelt