Just for clarification(unintentional pun!) I do not believe that a certain amount of folks filter their water because they are fearful of giardia, (which is extremely unlikely in the Whitney Zone, or most of the high sierra trails).
I am fearful of giardia (because I had it, so it isn't extremely unlikely to ingest those critters in the High Sierra). I thought it would not happen to me after so many summers of drinking unfiltered water. I also read everything about that stuff, and figured I was smart enough to save me the hassle and weight of a filter. Thing is, even if you don't plan on it, sometimes you will find yourself in a place that appears safe, you'll be thirsty and suddenly you're careless and drink what you should have left alone or at least boiled or treated.
Even after a spoiled trip due to giardia, I still don't own a filter; I will just try to be even smarter and avoid sub 8000 foot Yosemite streams. However, I will buy a filter for a October Muir Trail hike I am planning - during my last September trip it was not only difficult to find clean water sources, but water in general. With a filter I would have been able to use almost all surface water. Without filter I had to go through some dry stretches.
