Well, the basic case you set forth is exactly right. These days, several companies are making “individual” water-purifying systems. These look like an ordinary sport-style water bottle, but contain a water filter for removal of giardia, cryptosporidia, and other water-borne nasties. An example is the SafeWater G2 Personal Filter ($30; www.safewateranywhere.com). Some filters also add an iodine-charged anti-viral component, such as the Exstream Mackenzie ($50; www.exstreamwater.com).
I’ve used these gadgets, and they do seem to work well. That is to say, I drank water and didn’t fall ill afterwards. Admittedly, not the most scientific test on my part, but these products have all been rigorously tested and do perform well. They also save you carrying a separate filter and bring added convenience聴you just fill the bottle, go, and drink. Simple.
Still, there are drawbacks. For multiple-person parties, a single filter will weigh less than having everyone carry his or her own filter or bottle. It will cost less, too. Even for a solo hiker, a filter is the way to go in situations where you may not hit water frequently. With a traditional stand-alone filter such as the MSR Miniworks ($80; www.msrcorp.com) you can of course fill multiple water bottles or even a hydration bladder. With the solo bottles, you’re somewhat limited in terms of volume (of course, you can fill other bottles with untreated water and decant them into the filter-bottle as needed).
So, in conclusion, I’d surmise that for someone who hikes solo quite a bit, these bottles make perfect sense. For groups, maybe less so.