This is a sensitive topic, and I'm not entirely certain why. I guess because it's so personal and involves hygiene. And also because there's a large industry around water purification products and it's taken as gospel that water purification is required.
I think markskor provides the most rational response. In fact, this maps to what's preached in wilderness medicine conferences (my wife recently attended one in Squaw). Bottom line: people get all kinds of GI issues from many different sources. It's difficult, if not impossible, to attribute it so easily to giaradia introduced by water.
My rule of thumb: evaluate the water source. Is it a popular lake? Is it a creek with a popular lake or grazing/horsepacking upstream? Is it late season? Then I generally treat the water.
Is it a creek with little used trails upstream? Is it early season (or there's still lots of runoff)? Then I don't bother to treat the water. Why am I so liberal? Mainly because I have a strong stomach and rarely get ill. Other people are far more sensitive to stomach bugs, and are more cautious, as they should be.
Of course, my behavior is also influenced by my hiking partners. Some insist on always treating water. In that case, I go with what the party prefers. Not a big deal.
We always use iodine+vitamin C since all of our filters are clogged.