I am making this a "new post" because I took some pains to list some relevant links, and I think it is a potentially interesting, though certainly not new, topic for debate.
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The most widely known article debunking microbiology hazards in backcountry water was published in various updated versions over a period of years by Robert Rockwell. A version published in May 2002 is found here:

http://www.yosemite.org/naturenotes/Giardia.htm

Take a close look at the long list of citations at the bottom of the article. This is not just some loopy guy's opinion. A number of the cited articles are available on the Web.

Rockwell believes that water filters have no value in the Sierra Nevada, and by implication, also, in many other areas.

It's unhelpful to say that science is of no value because some scientists are liars, or that they make mistakes. Generically speaking, science is collaborative and depends on repeating the results obtained by others.

The data in this item http://www.wemjournal.org/pdfserv/i1080-6032-006-02-0162.pdf used reporting and data from 48 state health departments. To simply say that somebody, somewhere, might be a liar, or has "no experience" is completely unreasonable.

As for diseases other than giardiasis, typically one measures coliform bacteria as a proxy for infectious disease. So these measurements cover all your bases: http://www.yosemite.org/naturenotes/DerletWater.htm

Robert Derlet of UC Davis School of Medicine is slightly more conservative that Rockwell about the efficacy of backcountry water treatment, but in essence, he says the much same thing.

After he discusses the science, Derlet, who has made a study of Sierra Nevada water quality for a number of years, presents his personal bottom line:

"Should you treat your water to kill bacteria? This is a personal decision, and depends on the risk the individual wishes to take. I believe the risk is low and treat my water in only two situations: 1) below cow and sheep pastures; and 2) in slow flowing, warm streams immediately below places humans may be swimming or bathing."

I note that it ought to be easy to avoid these two types of sources specified by Derlet, while relying on other sources.

Myself, I own and use a water filter but whether I take it along or not depends on where I'm going. Though my outdoor resume is unavailable, let me mention that in nearly forty years of lots and lots of camping, for the most part, I've never treated water.