Well, I'd counter that you're using treatment as a first line of defense. I believe it should be used when there is a high degree of known personal risk.

One of the ways a body build immunities is by exposure to small amounts of the substance that cause an illness. There are many, many, people who are immune to Giardia variations that thrive where they live. ( one of many credible sources )

That is what I meant when I said drinking untreated water may help build your immunity to Giardia. Getting taken down hard with Giardia is almost always a sure sign of a weakened immune system.

There are many ways to strengthen your immune system that are well documented. Also well documented are many ways to weaken your immune system. Using chemical water treatments is a surefire way to reduce or stop your natural defenses against giardia, so is taking antibiotics.

I choose to try and keep my immune system strong with a selective diet, exercise, and if my diet is lacking, nutritional supplements made for that purpose, and by avoiding antibiotics and toxic chemicals like Chlorine Dioxide, which are known to weaken the immune system. I have never gotten sick drinking untreated water from clear running wilderness streams.

It's important to note that most Giardia infections are transmitted human to human, not by drinking water. Your hiking partner can be immune and still be a carrier and if you don't sanitize them too you're still at risk no matter how well you filtered your water.

In short, I'd rather work on building an immunity than preventing exposure.

You might call all that "some edumacated hunch", but there is well documented scientific research to back it all up and there are risks involved in ignoring it.