I've considered putting a bag on the bike, but I like having my stuff with me, and the backpack makes that easy. Plus, here in Phoenix, I will be pretty sweated up no matter whether I have the backpack on or not, and the pack has the added benefit of a constant source of hydration. I use a Camelbak, and having that tube available all the time for constant small drinks is the best and safest way in the heat. And it sure beats reaching down for a bottle, which is a long way on a 68cm bike.