As a Scout leader and weekend camp director for two years, I can tell you that the boys with the hydration packs drink more water. Yeah, the other boys can carry water bottles around - I have found a lot of water bottles (not empties) left behind after an activity and I've never found a hydration pack.

I've been dragging my old ALICE Pack (Medium) around for 20 years. While I can clip a 2 Qt canteen on it - my next pack will have a Camelbak pocket.

Ben