I guess my reply would be becuase they are convient.

A few weeks ago, I was helping to clear some roofs of snow around town. We get 6 feet of snow in a winter, but people build flat roofs, go figure.

Anywho, I had my little nalgene in my coat pocket, and I'd top that off from one of the big ones from my pack every so often. One of the guys with us had a small camelbak under his coat. I had started with HOT water in my big bottles, but it cooled pretty quickly in a pocket outside of the insulation of my coat. He had warm coffee in his jacket that stayed warm becuase it was under his insulation, and kept him warm becuase it was right over his kidneys.

I had to stop to drink. He didn't- sucker was sipping away going down a ladder!

So, am I trading in my Nalgenes for a bladder? Not a chance. But am I picking up a bladder? You bet. I've had to work in places that made my claustraphobia go into overdrive. But if I can stay properly caffinated, I'm able to control the urge to run screaming from an access tunnel better. It might be flat Dew or superstrong iced tea, but caffeine right handy without having to twist about in a tight place, and cool to.