You're a lot more likely to get sick from poor campsite hygiene than from drinking untreated water.  Giardia originally came from people, not beavers.  People are still the source you are most likely to pick it up from, too.

Just pay attention to where you get your water and you can leave the expensive, easily compromised filtering equipment at home.  I bring a bottle of treatment tablets just in case I am unable to get water from a source which meets my minimal standards.

That said, there's no way you'll get me to drink from Lakes 1-4 and several other insanely popular lakes without treating the water.  I don't fear the beavers too much.  I fear the two-legged disease carriers much worse.