^Like someone above said--you're more likely to stay better hydrated when you've got a straw right next to your mouth instead of down on your bike.

Not to say that people using bottles are dehydrated, or that there aren't other hydration pack options, but I have noticed the same thing. When I wear a camelbak, I drink without even thinking about it. When I've got a bottle, I only drink when I remember too (usually when I'm slowing down for some reason).