With the growing popularity of 29er mountain bikes and more complicated full suspension bikes, water bottle cage placement has taken a back seat to suspension components and frame geometries, leaving barely room for one bottle mount these days. (Smaller road bike frames also don't leave much room for cages.)
Just as bikes have changed, so have hydration packs -- they have advanced beyond a sack of water on your back. They now have extra features that make the ride more enjoyable and many (including this CamelBak that's half-off) have anti-microbial properties to inhibit the growth of bacteria.
Why should YOU consider a hydration pack?
++ It's healthier. Medical studies overwhelmingly point to improved athletic performance from a well-hydrated rider. Most riders are far more likely to keep up on the necessary liquid consumption when drinking from a hydration pack.
++ It can carry other gear you need. The CamelBak X.L.P. Pack has pockets to carry a tube, multi-tool, cell phone or some food, to make it more convenient than just a way to hydrate.
++ It is ideal for long rides and training. Considering the MS 150 Pedal to the Point, GOBA, or any other long ride? Hands-free hydration is easy and safer.
++ It can be used more places than on the bike. Today in Rocky River, a woman bought two -- one for her son for riding and one for herself for hiking.