I'm late to the party (nothing authoritative to contribute), but now that we're in the "summing up/what's done in practice?" stage of the thread, I'll chime in.
I agree that building immunity is important, and maybe the best defense (which is why I don't make use of any of the alcohol dispensers that are at the entry to every public building.) While ole Dan'l Boone had better quality water to drink, mostly, he probably also had a sturdier immune system from prolonged exposure. (Of course, no one's going to carve "D. Boon had diarrhea here" on a tree, so we may not have all the evidence available.)
In practice, I also tend to take the "belt and suspenders" approach to water purification: choose reasonably safe sources, but still purify. At 4 ounces and minimal effort (gravity filter), it's pretty cheap insurance.
Like someone else said, reasonableness and situation also play a role. When I've seen it bubbling up out of the ground, 2500 feet above and 20 miles away from the nearest village or farm, I will drink without purifying, maybe (I'm not immune to that temptation), but often I'll even purify then. And, given the choice between dehydration and drinking unpurified water, I'll drink the water.