I started carrying my Camelbak (off brand, but works the same) when I go on rides longer than a few minutes. I've found over the years that I need a pretty constant influx of quite a bit of water and the Camelbak is the most convenient way I've found to get a sip while riding.
The pack also has enough room for extra gloves, a light jacket, snacks, GPS, Spot emergency locator, and a few other odds and ends that I want with ME, not just on the horse. If I get tossed and Mr. Big decides to meet me in the next county, there are a few things I want with me, not him!
I've tried the water bottle in the pommel bag routine and it didn't work very well for me--although it'd probably work better now that I'm more comfortable sitting the horse! But that doesn't solve the problem of other stuff I want with me, anyway.