Two things to consider:
1. A healthy, non-immunocompromised adult can tolerate a certain degree contamination by the usual nasties (cryptosporidia, giardia, et alia). This is exactly why we evolved immune systems.
2. A bottle of water purification tablets weighs a fraction of nothing.
For my part, I've drank a lot of water directly from the source, and I've never had a problem. But I seem to have a pretty robust immune system, and for what it's worth, I only drink untreated water if it "seems clean" (which, I admit, is a terribly unscientific criterion for a scientist to use! heh). But I am also aware that each time I take a drink from a rushing mountain stream, I am playing a game of gastrointestinal Russian Roulette.