I'm not fond of ponchos in a windy environment like Whitney...they tend to flap around and let too much rain in.
Plus, a good waterproof shell is also excellent wind protection as the other posters have noted. For summer use, I found an inexpensive waterproof/breathable rain jacket at Burlington Coat Factory...about $40, as opposed to $100 from Campmor or $150 from REI. (Looks from the inside like a Gore-Tex clone, white inner layer under the nylon shell, plus a mesh liner to keep the waterproof membrane off your skin or base layers.) Plus, it's a good 12 oz. lighter than my Campmor shell that I use around here (St. Louis, MO) in the wintertime. Don't know how durable it is, but it worked fine for our Scout trek through Philmont (NM) the last couple of weeks. 100 miles in 11 days, needed a rain shell at least briefly almost every day...
My experience with rain pants is that I invariably end up as wet on the inside because of perspiration as I would have from the rain. I prefer fast-drying nylon zip-off pants...shorts when it's hot, long when it's breezy or cool, shed most of a light rain (or snow) and dry quickly if they do get wet.
Just my $0.02 worth...
