That's also one of the knocks on re-using the lightweight polyethylene bottles that sports drinks or bottled water typically come in. They're difficult to wash, and once you've wrapped your lips around the wet end of one, you've launched a little armada of bugs into a growth-friendly microclimate. But sure, I see them used a lot, and there's no reason not to do so. Myself, I don't find them all that durablea hard ding against a sharp rock while scrambling up a hillsidecould easily leave you without a water bottle. However, some people do wrap them in duct tape to help beef them up.
That said, spending $6 each on a Nalgene bottle or two remains a good investment and is hardly going to break the bank. The things last literally for years (and some believe them to be absolutely indestructible), so your actual cost is pennies per trip. Plus, they offer a host of advantages over your downmarket plastic bottle. They're much more rugged, for one thing. And the threaded top is compatible with many water filters. Finally, they're easier to clean. All in all, reason enough to stick a couple of them in your pack. If you're worried about the girth of some of the bigger bottles, check out the slimline N-Gen ($10; www.nalgene-outdoor.com) family of water carriers that come in snazzy colors like parrot green and fire-engine red.