Here is the collected thoughts I have on the topic:

Giardia is out there. When, where and how much is the question that will never be answered - there is no test we can do to verify, so the only completely safe thing to do is treat ALL WATER.

How do I know it is out there? I keep running into people who have had it - not just "got sick" but "went to a doctor, verified the diagnosis and had to have a bunch of antibiotics because it was giardia." This includes water rescue team members (who do not backpack, and therefore there is no "hygiene issue" to blame) who have been treated 2-3 times apiece for giardia, thanks to white water kayakers who go hike in miles and get into some mid elevation stream (Cherry Creek, the Kings River backcountry, and Dinkey Creek are but three central Sierra locations where they do this kind of thing, and sometimes things go wrong and they die - this happened just a couple years ago in Dinkey).

Do I know where it is? Nope. But I go up there 3 - 10 days per month. Odds are in my favor of exposure. At some point (don't know when, don't know where) conditions will be such that the little buggers may just hit critical mass just as I arrive at the stream to take a drink. So I'll keep ignoring comments about "overblown" and keep filtering, thanks all the same.

Someone who spins the wheel twice a year (the majority of the population I'd guess) and doesn't get sick has no impact on my belief that it's out there, because they don't go often enough to have that much of a risk. It's like fishing. If you stand in one place long enough, pitch the same lure every few minutes, eventually some dumb fish will swim by and bite - it doesn't mean the fishing in that spot is good, or that you're a good fisherman, or there are a lot of fish in the water, it just means you spun the roulette wheel often enough to get lucky. That's why rangers will tell you to treat the water. If enough tourists head out on enough trails to enough waterways.... some fraction of them will be unlucky, and some smaller fraction of them will have a good attorney.

I will always do my best to be safe. I'm the only one I have to take care of me. Depending on some decade old study or some edumacated hunch someone has is not good enough.