> In some circumstances,untreated water may be better than no water at all... Dehydration has serious consequences....
Well of course, all of this goes without saying, but it's situation dependent. If you're in 1st-world countries, giardia and crypto' are rather mild possible consequences, even if you can't go home on time. In 3rd-world countries, when you're facing the possibility of some rather nasty bugs, you need to think a little harder. Both before you go and when you get there.
But in the U.S., our backcountry water is fairly clean. Even the possibility of giardia and crypto is minimal. Still, even if it's present, that doesn't mean you will necessarily get it. If you consume 50 cysts, you probably will not get giardia. If you consume 100 cysts, you MAY get giardia. If you consume 150 cysts, you will probably get giardia. But giardia and crypto' NEED access to your digestive tract, so you can't get them by splashing wild water on your skin, eyes, ears, etc.
In any case, I routinely screen my water collection bag with a head-net before I filter (helps keep the filter clean, promote flow rate, and minimizes the need to backflush). I also lightly backflush my filter after every use. If I'm cooking, I always boil before I add the [dehydrated] food. If I'm drinking, and I have ANY qualms about purity, I have tablets in my survival kit or I'll boil. That's just routine behavior for me.
If I'm going to a riskier area, (and presuming we're talking about backcountry areas), I always know where the possible water sources are before I go out (but then I know how to find water sources wherever I am) and the likelihood of contamiation. I also top-off my water bottles every chance I get, even if I'm not empty. I always look for information on area water often by asking the Ranger Office, if there's one nearby, for ANY updates before I head-out. Etc., etc., etc.
Regardless, you always have to balance immediate need with later consequences. Unfortunately, human behavior dictates we are NEVER so concerned with later consequences (that's why we have a climate change issue!). This is what makes a PROPER wilderness survival education so valuable, it teaches you best practices you can rely on.